Decimal arithmetic logic unit for doubling or complementing decimal operand

ABSTRACT

A data processing system having a central processing unit (CPU) capable of performing binary and decimal arithmetic software instructions is described. The CPU includes a microprocessor which executes the binary arithmetic software instructions under firmware control. Also disclosed is an arithmetic logic unit (ALU) that functions with the CPU. The ALU has operand inputs to which are connected switched steering circuits that permit particular operands and zero operands to be selectively applied to any or all of the ALU operand inputs. This allows easy performance of special arithmetic functions such as adding a decimal operand to itself when converting the decimal operand to a binary operand, and to subtract a decimal operand from zero when complementing decimal operands.

RELATED APPLICATIONS

The following patent applications, which are assigned to the sameassignee as the instant application, have related subject matter and areincorporated herein by reference. Certain portions of the system andprocesses herein disclosed are not our invention, but are the inventionof the below-named inventors as defined by the claims in the followingpatent application.

Ser. No.: 537,910

Filing Date: Sept. 30, 1983 Title: Apparatus For Performing SimplifiedDecimal Multiplication By Stripping Leading Zeroes Inventors: J.Bradley, B. Stoffers, T. Staplin Jr., M. Widen

Ser. No.: 537,928

Filing Date: Sept. 30, 1983 Title: A Nibble and Word Addressable MemoryArrangement Inventors: Melinda A. Widen, John J. Bradley, George M.O'Har

Ser. No.: 537,902

Filing Date: Sept. 30, 1983 Title: Apparatus and Method for Converting aNumber in Binary Format to a Decimal Format Inventors: John J. Bradley,Brian L. Stoffers, Melinda A. Widen

Ser. No.: 537,992

Filing Date: Sept. 30, 1983 Title: An Integrating Illegal Decimal DigitDetection Apparatus for Supporting Decimal Arithmetic OperationsInventors: Thomas C. O'Brien, Melinda A. Widen, Theodore R. Staplin, Jr.Ming T. Miu

Ser. No.: 537,751

Filing Date: Sept. 30, 1983 Title: An Equal Nine Apparatus forSupporting Absolute Value Subtracts on Decimal Operands of UnequalLength Inventor: Brian L. Stoffers

Ser. No.: 537,991

Filing Date: Sept. 30, 1983 Title: An Arithmetic Logic Unit With OutputsIndicating Invalid Computation Results Caused By Invalid OperandsInventor: John J. Bradley, Theodore R. Staplin, Jr., Ming T. Miu, ThomasC. O'Brien, George M. O'Har, Brian L. Stoffers, Melinda A. Widen

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to data processing systems and more specificallyto a data processing system which provides for execution of decimalnumeric software instructions.

2. Description of the Prior Art

There are primarily two different methods employed within modern dataprocessing systems for representing numeric data.

It can be represented in a binary format in which each bit within a wordis given a weight of 2 raised to a power such that the least significantbit, when a binary ONE, represents 2 to the zeroth power, the next moresignificant bit, when a binary ONE, represents 2 to the first power andso on. Negative numers in a binary format may be indicated by a sign bitat either the beginning or end of the number or by performing a two'scomplement on the number.

The other method of representing numbers is to represent them in sometype of decimal format. The decimal format commonly used is to have eachdecimal digit represented by four or more bits with the bits beingbinary encoded to represent the values of 0 to 9 decimal. Representingnumbers in the decimal format has two disadvantages. First, as thenumber gets larger, more bits are required to represent a number in adecimal format than in a binary format. Second, performing decimalarithmetic operations is more complex and generally slower than binaryoperations because there are discontinuities at the boundaries betweendecimal digits which are not present between binary digits.

Numerous techniques for performing decimal arithmetic operations in dataprocessing systems are known in the prior art. Some techniques aredescribed in the book entitled, Digital Computer Design Fundamentals, byY. Chu, published by McGraw-Hill Book Company Inc., 1962 which isincorporated herein by reference. These techniques generally requirethat individual decimal digits be manipulated one digit at a time atsome point in performing a decimal arithmetic operation.

Therefore, in order to speed up decimal operations, what is needed aremethods to efficiently manipulate individual decimal digits within adata processing system and methods which reduce the number of digitswhich must be manipulated during any arithmetic operation.

OBJECTS OF THE INVENTION

Accordingly, it is the primary object of this invention to provide anarithmetic logic unit having multiple inputs to which operands mayselectively be applied to perform special arithmetic functions.

This invention is pointed out with particularity in the appended claims.An understanding of the above and further objects and advantages of thisinvention can be obtained by referring to the following descriptiontaken in conjunction with the drawings.

SUMMARY OF THE INVENTION

In prior art arithmetic logic units (ALUs) operands applied to theirinputs for processing are obtained from fixed sources, such as a memory,with little or no flexibility in applying particular operands toparticular ALU inputs.

In accordance with the present invention an ALU is taught havingswitched steering circuits connected to its inputs that permitparticular operands and zero operands to concurrently be applied to anyor all inputs of the ALU. This allows easy performance of specialarithmetic functions such as permitting a decimal operand to be added toitself when converting the decimal operand to a binary operand, and tosubtract a decimal operand from zero when complementing decimaloperands.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the method of the present invention is performed andthe manner in which the apparatus of the present invention isconstructed and its mode of operation can best be understood in light ofthe following detailed description together with the accompanyingdrawings in which like reference numbers identify like elements in theseveral figures and in which:

FIG. 1 is a general block diagram of a data processing system utilizingthe present invention;

FIG. 2 is a block diagram of the data manipulation area of themicroprocessor of FIG. 1.

FIG. 3 is a block diagram of a portion of the control area of themicroprocessor of FIG. 1;

FIG. 4 is a block diagram of the commercial instruction logic of FIG. 1which carries out the principles of the present invention;

FIG. 5 illustrates the firmware microinstruction word fields of themicroprocessor of FIG. 1 and the commercial instruction logic of FIG. 4;

FIG. 6 illustrates the clock phase relationship of one microinstructioncycle of the microprocessor of FIG. 1;

FIGS. 7A and 7B illustrate the position of 8-bit bytes and 4-bit nibblesin a 16-bit word of a data processing system utilizing the presentinvention;

FIG. 8A illustrates the format of a single or double operand basicsoftware instruction processed by the central processing unit of FIG. 1;

FIG. 8B illustrates the format of a commercial software branchinstruction processed by the central processing unit of FIG. 1;

FIGS. 8C-1 through 8C-3 illustrate the format of commercial softwarenumeric, alphanumeric and edit instructions with in-line datadescriptors, remote data descriptors and a combination of in-line andremote data descriptors;

FIG. 9 illustrates the format of the data descriptors used by thecommercial software instructions of the central processing unit of FIG.1;

FIGS. 10A through 10D are logic block diagrams of circuitry utilized inthe commercial instruction logic of FIG. 4 in accordance with thepresent invention;

FIG. 11 is a flow chart of the method used by the central processingunit of FIG. 1 to perform decimal addition and subtraction commercialsoftware instructions;

FIG. 12A is a flow chart of a prior art method of performing a decimalmultiply;

FIG. 12B is a flow chart of the method used by the central processingunit of FIG. 1 to perform a decimal multiply;

FIG. 13 is a more detailed flow chart of the method shown in FIG. 12Bwhich is used by the central processing unit of FIG. 1 to perform adecimal multiply commercial software instruction;

FIG. 14A is a flow chart of a prior art method of performing a decimaldivide;

FIG. 14B is a flow chart of the method used by the central processingunit of FIG. 1 to perform a decimal divide;

FIG. 15 is a more detailed flow chart of the method shown in FIG. 14B,which is used by the central processing unit of FIG. 1 to perform adecimal divide commercial software instruction;

FIG. 16 is a flow chart of the method used by the central processingunit of FIG. 1 to perform a commercial software instruction whichconverts a number in a binary format to a decimal format; and

FIG. 17 is a flow chart of the method used by the central processingunit of FIG. 1 to perform commercial software instructions whichconverts a number in a decimal format to a binary format.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The implementation of the embodiment illustrated in the drawings iseffected with a given arrangement of circuitry. However, it isunderstood that other logic arrangements may be employed in carrying outthe invention to adapt the invention to various types of dataprocessors. Accordingly, it is not intended to be limited to thespecific schemes shown in the drawings.

DATA PROCESSING SYSTEM OVERVIEW

Referring now to the drawings, FIG. 1 illustrates the overall dataprocessing system in which the present invention may be used. Moreparticularly, FIG. 1 shows a main memory subsystem 10, a centralprocessing unit (CPU) 20 and an input/output (I/O) unit 40. The mainmemory subsystem 10 consists of three metal oxide semiconductor modules12, 14 and 16. The three modules are interfaced to the central processorunit 20 and the input/output unit 40 via main bus 26. The main bus 26gives the capability of providing access to and control of all memorymodules and input/output units.

The central processing unit 20 executes word oriented softwareinstructions that operate on fixed and variable length fields. The basicunit of information in the central processor is a 16-bit word consistingof two 8-bit bytes (see FIG. 7A). A 16-bit word can also be broken intofour 4-bit nibbles (see FIG. 7B). These words of information are used ingroups of one, two or more for basic instructions (see FIG. 8A for anexample of one of the instruction formats) or fixed or floating pointbinary operands (data). These words are also used in groups of one ormore words for commercial instructions (see FIGS. 8C-1 to 8C-3). Bytesare also used in variable length fields as decimal or alphanumericoperands (data).

CPU 20 is comprised of microprocessor 30, monitor logic 22, Read OnlyStorage (ROS) 24 and commercial instruction logic 28. Microprocessor 30is an NMOS, 16-bit chip capable of arithmetic, logic, and controloperations, driven by a 48-bit external firmware microinstruction wordswhich in the preferred embodiment are contained in ROS 24. Themicroprocessor 30 design permits the execution of the CPU 20 basicsoftware instruction repertoire which operates on fixed and floatingpoint binary data. Commercial instruction logic (CIL) 28 is used inconjunction with microprocessor 30 to permit the execution of the CPU 20commercial software instruction repertoire which operates on decimal andalphanumeric data with the microprocessor 30 under the control of bits0-47 of a 56-bit external firmware microinstruction word contained inROS 24. As will be seen below, ROS 24 contains 2K (where 1K=1024) of48-bit microinstruction words which are used to execute the basicsoftware instructions of CPU 20 and ROS 24 also contains 2K of 56-bitmicroinstruction words which are used to execute the commercial softwareinstructions of CPU 20 with bits 0-47 controlling microprocessor 30 andbits 48-55 controlling CIL 28.

MICROPROCESSOR OVERVIEW

Microprocessor 30 is designed to directly control input/output (I/O) andmemory operation for ease in integrated system designs. Themicroprocessor 30 design permits greater conrol and integration by useof a 48-bit external firmware microinstruction word that provides truehorizontal microprogramming allowing up to 12 simultaneousmicro-operations per 48-bit microinstruction word. The microprocessor 30design also permits 8 external hardware interrupts which generatevectors to firmware microprogram routines as well as allowing 5 externalsoftware interrupts that are handled under firmware control. Inaddition, microprocessor 30 provides for 10 external monitor bitsoriginated in monitor logic 22 that are sensed and controlled bysophisticated test branch and major branch operations by logic whichmicroprocessor control area 36 which allows for sophisticated branchingoperations to be performed within the firmware. Four of these tenexternal monitor bits are set by commercial instruction logic 28 tocontrol test branch and major branch operations of microprocessor 30when it and commercial instruction logic 28 are used together to executea commercial software instruction of CPU 20.

Microprocessor 30 is comprised of 5 major internal hardware logic areasas shown in FIG. 1. The five major logic areas are the data manipulationarea 32 which includes the arithmetic logic unit (ALU), the memorymanagement unit (MMU 34), the control area 36, the processor bus 37, andinternal bus 38.

The processor bus 37 consists of 20 address/data lines, one memoryaddress violation line and three general purpose control lines.Processor bus 37 is connected to main bus 26 and is used to provideaddresses to the main memory 10 and input/output unit 40 and to receiveand send data to main memory 10 and input/output unit 40.

Internal bus 38 is the major path for communications of informationbetween the other four areas of the microprocessor chip. Internal bus 38is 20-bit wide. There are 12 sources of information to internal bus 38under control of the 11 micro-ops within the 48-bit microinstructionword. The ALU is the default source to internal bus 38 if none of theeleven defined micro-ops are used.

The data manipulation area 32 performs arithmetic and logic operationson data and does memory address generation. Data manipulation area 32 isdescribed in greater detail in reference to FIG. 2 below.

The control area 36 of microprocessor 30 is logically divided into 3areas: input latches for control, testable registers, and the nextaddress generation. Control area 36 is described in greater detail inreference to FIG. 3 below.

The MMU 34 section of microprocessor 30 is comprised primarily of: aregister file, a 12-bit address for base relocation, a 9-bit comparatorfor checking the size of a memory segment, several 2-bit ringcomparators for evaluating access rights to a given segment, and storageflip-flops for indicating potential memory violations. During any CPUgenerated memory address cycle, the MMU 34 translates the softwarelogical address containing a segment number, a block number and anoffset value presented by internal bus 38 into a physical address whichis placed on processor bus 37 which in turn is transmitted to mainmemory 10 via main bus 26.

As can be appreciated as the description so far, CPU 20 executessoftware programs, the instructions of which are fetched from mainmemory 10, and performs arithmetic and logical operations on data alsocontained in main memory 10. The software program executed by CPU 20 hasthe ability to manipulate general and base address registers that aresoftware visible and the current software instruction is pointed to by aprogram counter. These general registers, base address registers andprogram counter, which are visible to the software being executed by CPU20, are physically contained within the data manipulation area 32 ofmicroprocessor 30.

Detailed operation of CPU 20 of FIG. 1 is controlled by microprocessor30 under the control of firmware microinstructions stored in ROS 24.Each location in ROS 24 can be interpreted as controlling onemicroprocessor machine cycle. As each location of ROS 24 is read, thecontents are decoded by control area 36 resulting in a specificoperation within microprocessor 30. By grouping ROS locations, firmwaremicroinstruction sequences are obtained that can perform a specificoperation or software instruction associated with CPU 20. As eachsoftware instruction is initiated, certain bits within the operationcode field of the software instruction are used to determine thestarting address of the firmware microinstruction routine containedwithin ROS 24. The testing of certain flip-flops which are set or resetby software instruction decoding done by microprocessor 30 allow themicroprocessor to branch to a more specific firmware microinstructionsequence within ROS 24 when necessary. When a commercial softwareinstruction is encountered, microprocessor 30 branches to that portionof ROS 24 which contains 56-bit microinstruction words so that bits 0-47are used to control the operation of microprocessor 30 and bits 48-55are used to control the operation of commercial instruction logic 28.CIL 28 is described in greater detail with reference to FIG. 4 below.

Connected to main bus 26 is a input/output unit 40. The input/outputcontroller 42 is that portion of the input/output unit 40 whichcompletes a data path from a peripheral device 44 to main memory 10 viamain bus 26. I/O controller 42 provides a path through which theperipheral commands are initiated, in addition to controlling resultingdata transfers.

The microprocessor 30 interfaces with other CPU 20 logic by means ofinterface signals. In the preferred embodiment, the interface signalsare divided into four groups according to pin assignment onmicroprocessor 30 and the phase relationship of the clock.

The microprocessor clock is the primary element of the interface and itproduces a Phase A and a Phase B signal as shown in FIG. 6.

The phase relationship between Phase A and Phase B determines thefunctions of the interface lines because there are 129 signals(excluding power, ground, and clock timing) shared among 57 input/outputpins of microprocessor 30 of the preferred embodiment.

Phase A signal is used by the microprocesor 30 and the system to reversethe direction of the processor bus 37 drivers. When Phase A is high (abinary ONE), the 48 ROS data lines from read only storage 24 and thefive option lines are inputs to the microprocessor 30. When Phase A islow (a binary ZERO), all other shared signals are either inputs oroutputs to microprocessor 30.

Phase B signal is used to latch the signals that were gated with PhaseA. When Phase B is going low, the ROS data and options are latchedinternal to the microprocessor 30. When Phase B is going high, all othershared signals are latched internal to the microprocessor 30.

There are four signal groups defined by the microprocessor 30 interface.Each group, except group 1, consists of signals that have common pinassignments and clock phase relationship shown in FIG. 6.

Group 1 consists of nine interface lines that have unshared signals.They are: three voltage, two ground, and four timing signals.

Group 2 consists of 23 interface lines which control 69 signals, five ofwhich are unused. During Phase A high, the 23 lines represent 21 ROSinput data bits and two input option bits. During Phase A low, the 23lines are bidirectional representing inputs of 16 data bus bits, 3control signals, and 4 unused signals; or outputs of 20 address/data busbits and 3 control signals.

Group 3 consists of 19 interface lines which control 38 input signals.During Phase A low, the 19 lines represent 4 software interrupt signals,8 hardware interrupt signals, and 7 monitor bits. During Phase A high,the 19 lines represent 14 ROS input data bits, 3 input option bits, and2 unused signals.

Group 4 consisting of 13 interface lines which control 26 signals.During Phase A low, the 13 lines represent 12 ROS output address bitsand an error signal. During Phase A high, the 13 lines represent 13 ROSinput data bits.

MICROPROCESSOR DATA MANIPULATION AREA

Referring now to FIG. 2 which illustrates the data manipulation area 32of FIG. 1 in greater detail. In FIG. 2, the number next to the upperright hand corner of the blocks represents the number of bits ofinformation contained in the register represented by the block.

The data manipulation area, as shown in FIG. 2, performs arithmetic andlogical operations on data and memory address generation. It is composedof the following eight elements: P-register 80, arithmetic logic unit(ALU) 83, G-register 84, a shifting mechanism (not shown in FIG. 2),Q-register 85, indicator register (I-register) 86, register file 82, andM-register file 81.

The P-register 80 is a 20-bit memory address register that containseither the program counter or a logical memory address. The P-registercan be loaded from the internal bus 38, incremented or decremented, andthe output transferred to the internal bus under firmware control byvarious microinstruction commands.

The G-register 84 is 20 bits wide and is used to hold addresses for ALUoperations. It can be loaded from the internal bus 38 under firmwarecontrol by a microinstruction command and the output is sent to theB-port of ALU 83.

The Q-register 85 is a 16-bit register that provides operand shifts, andholds secondary operands for ALU 83. It can be loaded from the internalbus under firmware control by a microinstruction command and the outputis sent to the B-port of the ALU 83. Q-register output can be signextended to 20 bits for ALU operations. The load Q-registermicroinstruction command is used for loading from internal bus 38 whenshifting microinstruction commands are not used.

The register 82 can be loaded from the ALU 83 output or the internal bus38. When loaded from internal bus 38, data comes direct, or with byte 0(the left most 8 bits) swapped with byte 1 (the right most 8 bits) of a16 bit word, or with the right most 4 bits from internal bus 83 beingloaded into the left most 4 bits of a 20 bit register in register file82. Two write lines control the loading of registers in register file82: one loading the right most bits 0 through 15 in 16-bit or 20-bitregisters, the other loading the left most bits 0D through 0A in 20-bitregisters. The register file 82 output feeds the A-port of the ALU 83.The register address is taken from either the ROS data register 65(RDDT) or the information in the F-register 51 (see FIG. 3).

During Phase A high time, the address for the selected register file isgenerated under control of the register file address field of themicroinstruction (see FIG. 5). The selected register file is read outduring Phase A high and its contents are latched in an output register(not shown in FIG. 2) during Phase A low. This output register is thenavailable to two possible destinations, the A-port of ALU 83 (undercontrol of the ALU control field of the microinstruction, RDDT bits 18through 22) and the internal bus 38 (under control of special controlfield of the microinstruction, RDDT bits 35 through 41) (see FIG. 5).

The selected register file is written near the end of Phase A low time.The general command for writing the register file is supplied by RDDTbit 26 (register file load). This general command can be modified bycertain special control field subcommands.

The register file description that follows is in terms of softwarevisible registers of CPU 20.

There are seven 16-bit software addressable data registers, R1 throughR7. They can be loaded from or stored into memory on either a word orbyte basis. Each register can be used as operands in arithmetic,logical, and compare operations. R0 is a 16-bit register that is used tohold a copy of the executing software instruction. There are sevensoftware-addressable base registers, B1 through B7. They are 20 bitswide and can be used to hold main memory addresses. B0 is a 20-bitworking register used when CPU 20 is in a maintenance panel mode. Six16-bit (DW1 through DW5) and five 20-bit (AW1 through AW5) workregisters are available for temporary storage of information duringfirmware operations. The 16-bit registers are normally used to hold dataand the 20-bit registers are normally used to hold memory addresses. AW1is normally used as the "effective address" storage element for thecurrently executing software instruction. The T-register is the stackaddress register and is 20 bits wide. System keys and processor securitykeys are contained in the 16-bit S-register.

field defines the CPU identification number to allow for multiprocessorsystems. A 6-bit level field defines the interrupt priority on which theCPU is currently operating. Zero is the highest and 63 the lowestpriority. The history register (H-register) is 20 bits wide. It containsthe history of the program counter under firmware control. The remotedescriptor base register (RDBR) is used by the commercial instructionlogic 28. It is 20 bits wide. The I/O data register is a 16-bit dataworking register that can be used for temporary storage during I/O datatransfer operations.

ALU 83 has full 16- or 20-bit capabilities. Overflow and carry functionsare generated out of both the 16th and 20th bit positions. The 16-bitcapability of the ALU is normally used when handling data, while the20-bit capability is normally reserved for address modifications andtransfers. The ALU 83 has two ports for operand inputs, the A-port andB-port. The A-port can accept either a 16-bit or 20-bit register fileinput. In the case where the register file selected is a 16-bit widedata register, the most significant bit is sign extended to 20 bits. TheA-port can also select a value of zero as its inputs as specified by theALU control field of the microinstruction (RDDT bits 18 through 22) (seeFIG. 5). The B-port can select as its inputs: 20 bits from theG-register, or 16, 8 or 7 bits right justified from the Q-register. Themost significant bit of the field selected from the Q-register port canalso select a value of zero as its input.

The output of the ALU can be directed to either internal bus 38 orregister file 82. When directed to the register file, its path can bedirect (by special control field subcommands) or shifted left or rightone bit position (also under control of special control field--allregister file shift subcommands). The ALU output is directed to theinternal bus 38 whenever there is no microcommand called to source theinternal bus 38 (it is the default source for the internal bus 38).

The carry and overflow conditions for both 16- and 20-bit operations arestored in temporary flip-flops for testing using test branchmicrocommands during the following cycle.

The microprocessor 30 has the ability to perform various shiftoperations (i.e., open/closed, arithmetic/logical, left/right) on either16-bit or 32-bit operands. Sixteen-bit data shifting can be accomplishedin one of two ways. The first takes place from output of the ALU 83 intothe register file 82 and the second takes place in the Q-register 85.These two operations can be concatenated to perform the 32-bit datashift operations.

Three shift microcommands are used to implement the software shiftinstructions of CPU 30. The shift microcommands are combined with theF-register 51 (see FIG. 3) decode (the F-register contains the softwareinstruction word for this operation) to determine the shift type,direction, and necessary filler bits. The following Table 1 shows thedecoded F-register bits and the corresponding shift type. In Table 1 theabbreviation "LE." means "less than or equal to" followed by the maximumnumber of bit positions that can be specified to be shifted, and theabbreviation "GE." means "greater than or equal to" followed by themaximum number of bit positions that can be specified to be shifted.

                  TABLE 1                                                         ______________________________________                                        F-Register Shift-Type Decoding                                                F-Register Bits                                                               8 Through 11 Shift Type                                                       ______________________________________                                        0            Single open left shift                                           1            Single closed left shift                                         2            Single arithmetic left shift                                     3            Double closed left shift                                         4            Single open right shift                                          5            Single closed right shift                                        6            Single arithmetic right shift                                    7            Double closed right shift                                        8            Double open left shift (LE. 15)                                  9            Double open left shift (GE. 16)                                  A            Double arithmetic left shift (LE. 15)                            B            Double arithmetic left shift (GE. 16)                            C            Double open right shift (LE. 15)                                 D            Double open right shift (GE. 16)                                 E            Double arithmetic right shift (LE. 15)                           F            Double arithmetic right shift (GE. 16)                           ______________________________________                                    

F-register bits 1 through 3 contain the R-register to be shifted, andF-register bits 12 through 15 or 11 through 15 contain the number ofpositions to be shifted. When F-register bit 8 equals a binary ZERO, thenumber of positions to be shifted is determined by F-register bits 12through 15; and when F-register bit 8 equals a binary ONE, the number ofpositions to be shifted is contained in F-register bits 11 through 15. Aspecial case exists when the count field contains a value of zero. Inthis case, the number of positions to be shifted is contained inregister file location R1 (general purpose register).

When a double word is selected (i.e., F-register bits 8 through 11equals a hexadecimal 3, 7, 8, 9, A, B, C, D, E, or F), then F-registerbits 1 through 3 must equal a 3, 5, or 7. This is necessary because itrequires a combination of an implied even-numbered register and anexplicitly addressed odd-numbered register to perform a double-wordshift operation. When register R3 is explicitly addressed, register R2is the implied addressed register. The even-numbered register containsthe most significant bits of the double word.

The I-register is eight bits wide, containing various single bitindicators in the following format:

Bit 0--Overflow indicator (OV): It is set when any of the data registersR1 through R7 overflow; e.g., when a 16-bit arithmetic result producedis larger than the capacity of the register (under op-code control, outof ALU).

Bit 1--Always a binary ZERO.

Bit 2--Carry indicator (C): This is set when the logical capacity of aregister is exceeded. The carry indicator is generated from the ALU.

Bit 3--Bit test indicator (B): This bit gives the state of the last bittested (primarily for bit test operations).

Bit 4--Input/Output indicator (I): It indicates whether the last I/Ocommand was accepted by the I/O controller.

Bit 5--Greater than (G) indicator.

Bit 6--Less than (L) indicator.

Bit 7--Unlike sign (U) indicator.

The G, L and U indicators are controlled by microcommands during thecompare instructions and contain the result of the last compareoperation executed. Typically, the comparison involves a register and aword from memory. The indicators show whether the register contents aregreater than or less than the memory word.

The I-register can also be loaded from the right byte of internal bus38, and its output goes to the right byte of internal bus with the leftbyte of the internal bus being set to binary ZERO bits.

The microprocessor 30 has a mode register (M-register) file 81 that haseight registers, each eight bits wide (8 by 8). This file 81 can beloaded from the internal bus 38 using a microcommand or sourced to theinternal bus 38 using a second microcommand. When using either of thesemicrocommands, the M-register address is supplied by the leastsignificant three bits of the register file address register. Whenneither of these microcommands are called, the M-register addressdefaults to location 1 (M1 register).

M-register bits 1 through 7 of any M-register are testable, with theregister file address field of the microinstruction (see FIG. 5)selecting the desired M-register, and the desired bit within theregister being selected by a code of 1 through 7 in F-register bits (0through 3). This causes the setting of a temporary flip-flop when abinary ONE is detected. Bit 0 of the selected M-register is sampled ineach cycle by another temporary flip-flop. Testing of these temporaryflip-flops is accomplished on the following cycle by using twosubcommands.

The various mode registers (M0-M7) are settable by software instructionsso that a software program can specify how certain conditions are to behandled when they arise during the program's execution. For example,setting bit 0 in M1 to a binary ONE indicates that all software branchesand jumps are to cause a trap to an entry of a software routine thatwill trace the program's execution. Bits 1 to 7 in M1 are used tocontrol the trapping if an overflow occurs in the corresponding dataregister R1 to R7 during arithmetic operations. Bit 0 of M3 controls thetrapping of overflow when a commercial software instruction is executedand for bit 1 of M3 control trapping if truncation occurs during theexecution of a commercial software instruction.

MICROPROCESSOR CONTROL AREA

Referring now to FIG. 3 which illustrates the control area 36 of FIG. 1in greater detail. Control area 36 contains additional logic andcircuitry, but for the purposes of the invention, the logic has beenlimited to that shown in FIG. 3 which is primarily concerned withaddressing read only storage 24. In particular, control are 36 alsoincludes the microinstruction decode logic (not shown in FIG. 3) whichcontrols the enabling and gating of the logic elements and registers ofmicroprocessor 30. FIG. 3 also illustrates internal bus 38, monitorlogic 22 and read only storage (ROS) 24.

In FIG. 3, the number next to the upper right hand corner of the blocksrepresents the number of bits of information contained in the registerrepresented by the block.

ROS 24 may be a read only memory (ROM) or a random access memory (RAM)or any other form of memory device capable of holding firmwaremicroinstructions. The ROS 24 contains the firmware microinstructions(or control words) which are used by microprocessor 30 and commercialinstruction logic 28 to control the operation of central processing unitand more particularly to execute the software instructions of CPU 20.For each microprocessor machine cycle, a control word is fetched out ofROS 24.

ROS 24 is coupled to ROS data register 65 which receives bits 0-47 ofthe microinstruction word fetched from read only storage 24. Eachmicroinstruction contains an address portion and a command portion. Theaddress portion in the microinstruction word identifies the address ofthe next location to be read from read only storage 24 which will be thenext microinstruction to be executed by microprocessor 30. The commandportion of the microinstruction identifies the operations to beperformed by the microprocessor 30 and commercial instruction logic 28during the execution of the current microinstruction.

The address portion of the microinstruction word may be contained in apredetermined number of bits, for example, in the preferred embodimentit is contained in bits 0 through 12 of the microinstruction word (SeeFIG. 5). The command portion of the microinstruction may also becontained in a predetermined number of bits, for example, in thepreferred embodiment it is contained in bits 13 through 47 which controlthe operation of microprocessor 30 and in bits 48 through 55 which, whenpresent, along with bits 35 through 37 control the operation ofcommercial instruction logic 28 (see FIG. 5). The command portion may befurther broken down into a number of fields which comprise subcommandsof the microinstruction.

Before describing the microinstruction word in greater detail withrespect to FIG. 5, the other elements of FIG. 3 will be described.Monitor logic 22 provides status information with respect to CPU 20 andis loaded into test flip-flops 50 such that the status may be tested bythe firmware. In addition to holding ten bits of dynamic statusinformation from monitor logic 22, test flip-flop 50 hold five bitswhich sample the status of various CPU options. The CPU option bitsshould be thought of as static in nature and indicate whether or not aspecific hardware option is present or not within the data processingsystem. For example, one CPU option bit indicates whether or not thecommercial instruction logic 28 is present in the system. In addition,test flip-flop 50 contains four control flip-flops (CF1-CF4) which areavailable to be set or reset or to have a bit transferred under controlof the firmware. These four control flip-flops are testable by thefirmware. There are also ten temporary flip-flops in flip-flops 50 whichare loaded during each firmware cycle with dynamic information such aswhether there has been a carry or overflow from bit 16 of the ALU or acarry or overflow from bit 20 of the ALU or whether certain bits on theinternal bus 38 are equal to 0, etc. These ten temporary flip-flops arealso testable by firmware.

The F-register 51 is a 16-bit instruction register that is loaded forminternal bus 38. All bits of the F-register are testable by firmware.The low-order four bits of F-register 51 also constitute the low-orderfour bits of the five-bit counter F-counter 52. F-counter 52 is afive-bit counter that can be loaded form internal bus 38. F-counter 52can be incremented or decremented. The four low-order bits of F-counter52 are also decoded such that a 16-bit mask can be placed on internalbus 38 under firmware control.

There are five possible conditions that can cause a software interrupt.These conditions are latched in software interrupt register 53. Softwareinterrupt prinet 54 prioritizes these conditions and generates avectored address for input into major branch logic 57.

The next address generation section 55 of control area 36 contains thelogic necessary for sequencing the read only storage (ROS) 24. Testbranch logic 56 is used to test 64 test conditions which can result in a2-way branch address for ROS address register 63. These 64 testconditions are testable under firmware control and with the output ofthe test branch logic 56 being one bit of information into addressmultiplexer 1 60. Inputs to test branch logic 56 are provided by testflip-flops 50, F-register 51 and F-counter 52.

Major branch logic 57 provides 15 major test branch matrixes. Themajority of the inputs to these matrixes are from F-register 51 (invarious combinations). Other inputs are from the monitor and option bitsof test flip-flops 50. The output of major branch logic 57 is four bitsof address information which is provided to address multiplexer 1 60.

Register 58 provides the bits of information that correspond to the tenpossible conditions that can cause a hardware interrupt. Hardwareinterrupt prinet 59 prioritizes these ten possible conditions andproduces a four-bit output that is used by address multiplexer 2 62 toproduce the 12-bit vectored hardware interrupt address when one of thesten possible conditions occur.

The output of address multiplexer 1 60 provides the 12-bit nominal nextaddress which will be loaded into ROS address register 63 and used tofetch the next microinstruction from ROS 24. This 12-bit address isnominal in the sense that this nominal next address will be used as thenext address only if a hardware interrupt does not occur. A hardwareinterrupt will not occur if no hardware interrupts are pending or ifpending hardware interrupts are inhibited by the setting of theinterrupt inhibit bit within the microinstruction word (see bit 34 inFIG. 5).

Address multiplexer 2 62 is used to select between the 12-bit nominalnext address generated by multiplexer 1 60 and the vectored hardwareinterrupt address that is produced by combining the 4-bits from hardwareinterrupt prinet 59 with 8 leading 0 bits. The output of addressmultiplexer 2 62 is the 12-bit next address which is loaded into ROSaddress register (RAR) 63. The output of RAR 63 is used to provide theaddress of the next microinstruction to be fetched from ROS 24. Theoutput of RAR 63 is also input to ROS address history register 66.

ROS address history register 66 is provided so that early in theexecution of the current microinstruction contained in ROS data register65, while the next microinstruction address is being developed andtransferred to ROS address register 63, ROS address history registerholds the address of the current microinstruction. This currentmicroinstruction address is used in developing the next microinstructionaddress if the current microinstruction calls for its use. The currentaddress from ROS address history register 66 is also used after beingincremented by incrementer 64 as the return address frommicrosubroutines and hardware interrupt service routines.

Incrementer 64 increments by a predetermined number (e.g., by 1 in thepreferred embodiment) the address contained in RAR address historyregister 66. Incrementer 64 is a 12-bit incrementer which will be usedto source the return address stack 70 via return multiplexer 61 during aPUSH microcommand. The output of incrementer 64 is also used to providethe next ROS address value to RAR 63 for INC and INCK mircocommands viaaddress multiplexer 1 60 and address multiplexer 2 62. The INCmicrocommand specifies that the next ROS address is to be the currentROS address incremented by one and the INCK microcommand specifies thatthe next ROS address value is to be the current address value plus 1 andin addition a constant, as specified in other unused address field bitswithin the microinstruction, is to be placed on internal bus 38.

Return address stack 70 is a 4 by 12-bit last in first out (LIFO) arrayused for storing the return addresses of subroutines and hardwareinterrupts. Return address stack 70 is initialized to hexadecimal valueof 001 during clear time and its bottom location is set to 001(hexadecimal) during each POP (return) microcommand. A PUSH microcommandcauses the top of return address stack 70 to be sourced by the output ofincrementer 64. A hardware interrupt causes the top of stack 70 to besourced by the output of address multiplexer 1 60, which is the nominalnext address. Incrementer 64 transfers to return address stack 70 theincremented address history from ROS address history register 66 whenone subfield of the next command portion of the ROS data register 65specifies a PUSH microcommand. This PUSH microcommand enables thestoring of the return address of the microprogram microinstruction thatis being executed while branching to a microprogram subroutine.

In response to the PUSH microcommand, incrementer 64 provides that theincremented current ROS address from ROS address history register 66 toreturn address stack 70, which comprises a plurality of registers 71through 74. Functionally, return address stack 70 is a push down storagedevice which comprises a plurality of work registers arrayed in acolumn. The only output from the stack is from top register 71 which isconnected to address multiplexer 1 60. The only inputs return addressstack 70 are from the top and bottom. When an address is pushed ontostack 70, it goes into register 71 after the other addresses already inthe stack are pushed down the column one register. As an address isremoved from the column (popped up), it is provided by top register 71to address multiplexer 1 60 and each address stored in return addressstack 70 moves up one hardware register in the column. During this popoperation, the bottom register 74, which is vacated, is loaded with theaddress 001 (hexadecimal). The stack can be visualized as a deck ofcards, wherein access to the cards of the deck is only possible byadding or removing cards one at a time to or from the top of the deckand wherein a predetermined card (hexadecimal value 001) is added to thebottom of the deck as each card is removed from the top of the deck.

Return address stack 70 thus stores the incremented current address asis provided from ROS address history register 66 when the executingmicroprogram branches to a subroutine. In addition, return address stack70 stores the nominal next address output by address multiplexer 1 60whenever a hardware interrupt occurs which vectors the execution of thefirmware to a predetermined location within ROS 24 as determined by theparticular hardware interrupt that has occurred. These addresses storedin return address stack 70 point to the next step of the microprogramwhich would have been executed except for the occurrence of a branch toa microprogram subroutine or a branch to a microprogram interrupthandling routine. Since these addresses will be stored when a branch toa microprogram subroutine occurs, or when a hardware interrupt occurs,the addresses in return address stack 70 will, upon the execution of thelast microinstruction in a subroutine or hardware interrupt handlingroutine return the microprogram to the proper sequence.

FIG. 5 illustrates the firmware microinstruction word fields ofmicroprocessor 30 of the preferred embodiment. This microinstructionword is comprised of 56 bits (bits 0-55). Bits 0-47 which controlmicroprocessor 30 will be discussed now with reference to FIG. 3 andbits 48-55 which control commercial instruction 28 will be discussedlater with reference to FIG. 4. Bits 0 through 12 are used as the ROSaddress field, bits 13 through 17 are used to select registers in theregister file, bits 18 through 22 are used to control the arithmetic andlogic functions of the ALU and the inputs to its ports, bits 23 through25 are used as bus control, bits 26 through 30 are used as a registermodification field, bits 31 through 33 are used as memory managementunit control, bit 34 is used to inhibit the occurrence of a hardwareinterrupt and bits 35 through 47 are used as a special control field.

The special control field (RDDT bits 35 through 47) is used to modify aswell as supplement certain of the other fields in the microinstructionfirmware word. The special control field provides up to threesimultaneous microcommands during a given microcycle. The specialcontrol field is divided into 4 subfields (A through D) as illustratedin FIG. 5. With the interpretation of some of the subfields dependentupon the contents of other subfields.

The 48 bits of the microinstructions are loaded into the ROS dataregister 65 at the beginning of the execution of the microinstruction.These 48 bits are referred to as signals RDDT00 through RDDT47.

The ROS address field contains 13 bits (RDDT00 through RDDT12) and isused to generate the address of the next firmware step in a givenmicroprogram sequence. The method for generating this next address isdefined by the first five bits of the ROS address field as shown belowin Table 2.

                  TABLE 2                                                         ______________________________________                                        ROS Address Field Microoperations                                             RDDT Bits                                                                     0    1     2       3   4     Operation                                        ______________________________________                                        1    X     X       X   X     Jump                                             0    1     X       X   X     Test Branch                                      0    0     1       X   X     Major Branch                                     0    0     0       1   X     Increment With Constant                          0    0     0       0   1     Increment Without Constant                       0    0     0       0   0     Return (POP microcommand)                        ______________________________________                                    

A PUSH microcommand can be used in conjunction with any of the firstfive operations listed in Table 2. The PUSH microcommand, when used incombination with a jump or branch microcommand, allows themicroprogrammer to store away into return address stack 70 a returnaddress to which the microprogrammer will wish to return upon completionof the subroutine which was branched to. To facilitate the storing awayof this return address by the person writing a microprogram, the PUSHmicrocommand pushes the contents of the ROS history address register 66incremented by 1 by incrementer 64 onto the top of return address stack70. The return (POP) microcommand is then used by the microprogrammer asthe last firmware step of the called subroutine to return to the firstlocation after the microinstruction which called the microprogramsubroutine.

One exception to the next address generation being defined by the sixoperations described in Table 2 is that of a hardware interrupt. When ahardware interrupt is initiated, the next ROS address will be providedas a hardware vector and the ROS address generated by the ROS addressfield of the present firmware word will be placed on the top of thereturn address stack 70 by the output of address multiplexer 1 60 beingselected as the output of return multiplexer 61 and pushed onto returnaddress stack 70. If a PUSH microcommand (as specified by specialcontrol field C in bits RDDT42 through RDDT44 in conjunction with aspecial coding of subfields A and B) is used in conjunction with one ofthe first five operations shown in Table 2, then the microprogrammermust also inhibit hardware interrupts by specifying that RDDT bit 34 isa binary ONE in order to prevent a conflicting push operation beingperformed as the result of the occurrence of a hardware interrupt. Sincea hardware interrupt can occur (normally by the occurrence of anexternal asynchronous event) at any time prior to the completion of agiven firmware microprogram sequence, special consideration must begiven to allow for the occurrence of a hardware interrupt.

The branching capabilities of the six operations defined in Table 2 arereferred to as page branching and bank branching. A page is defined as64 consecutive memory locations within ROS 24 and a bank is defined as1024 memory locations within ROS 24 (16 Pages). The branch boundariesfor the test branch operation is restricted to any location within apage. The branch boundaries for the major branch are restricted to anylocation within a bank. The remaining four operations of Table 2 arecapable of branching or incrementing from one bank to another.

The jump operation is the only next address generation method of Table 2that allows a branch to any of the possible 4096 locations of ROS 24.This is accomplished by providing, within the ROS address field, a12-bit direct address of the next firmware microinstruction to beexecuted.

When RDDT bit 0 equals a binary ONE, RDDT bits 1 through 12 of thepresent ROS data word (firmware microinstruction) as contained in ROSdata register 65 are delivered directly to ROS address register 63 viaaddress multiplexer 1 60 and address multiplexer 2 62 as the nextaddress in the firmware microprogram, assuming of course, no interveninghardware interrupt occurs. Should a hardware interrupt occur, thisnominal next address would be pushed onto the top of return addressstack 70 and the generated hardware interrupt vector address, asgenerated by concatenating eight high order binary ZERO bits with thefour bit output of hardware interrupt prinet 59, as output by addressmultiplexer 2 62 will be loaded into ROS address register 63 as the nextROS address.

A PUSH microcommand (as defined by special control field C) can be usedalong with the jump operation. Hardware interrupt inhibit bit RDDT34must be set to a binary ONE to inhibit hardware interrupts when the PUSHmicrocommand is used in conjunction with a jump operation. During a PUSHmicrocommand, the current ROS address held in ROS address historyregister 66 is incremented by 1 and pushed onto the top of returnaddress stack 70 by return multiplexer 61 before the next address asspecified in RDDT bits 1 through 12 is loaded into ROS address register63.

The test branch operation of Table 2 is a two-way branch using theresult of one of 64 test conditions specified as part of the ROS addressfield in bits 3 through 8. All test branches are restricted to branchingwithin the current page, that is, the next ROS address generated as aresult of the test will always be one of two locations (depending uponthe results of the test: i.e., true or false) eight locations apart butwithin (64 locations) currently being address by ROS address register63. That is, depending upon the results of the test branch, the nextmicroinstruction will be fetched from the location determined by takingthe six high-order bits from the current ROS address (from ROS addresshistory register 66) concatenated with the six low-order bits which comefrom bits 2, 9, 10, 11 and 12 of the ROS address field which are usedfor bits positions 7, 8, 10, 11 and 12 respectively within the nominalnext ROS address and with the result of the test (1 or 0 correspondingto true or false respectively) being used as bit position 9 within thenominal next ROS address.

This nominal next ROS address composed of the bits as described above,generated as the result of the test branch as specified in bits 0through 13 in the current firmware microinstruction word, is the nextROS address assuming no intervening hardware interrupt occurs. Should ahardware interrupt occur, this address is placed on the top of returnaddress stack 70 and the generated hardware interrupt vector addressreplaces it as the contents of the next ROS address in ROS addressregister 63.

The PUSH microcommand can also be used along with test branchoperations. As indicated above, hardware interrupts must be inhibited(by setting RDDT bit 34 equal to a binary ONE) if the PUSH microcommandis used. If the PUSH microcommand is used in conjunction with a testbranch operation, the current ROS address, which is the address of thetest branch microinstruction (from ROS address history register 66)incremented by 1, will be placed on the top of return address stack 70and the microprogram will branch to the nominal next address asdetermined by the output of test branch logic 56.

The major branch operation is a 16-way branch using the results of 15test groups specified as part of the ROS address field in bits 5 through8. All major branches are restricted to branching within the currentbank (1024 locations). That is, the nominal next ROS address generatedas a result of the major branch test will always be in one of 16locations (depending on the output of the major branch matrix) 16locations apart but within the bank (1024 locations) currently beingaddressed by ROS address history register 66. The nominal next ROSaddress is generated by taking bit 0 and 1 from the current ROS addressand using them as bit 0 and 1 in the nominal next ROS address and takingbits 3, 4, 9, 10, 11 and 12 from the ROS address field and using them asbits 2, 3, 8, 9, 10 and 11 in the nominal next ROS address respectively.In addition, bits 4 through 7 in the nominal next ROS address aredetermined by the four-bit output by major branch logic 57.

The nominal next ROS address as described above is generated as theresult of the major branch operation specified in bit 0 through 12 ofthe current firmware microinstruction word contained in ROS dataregister 65 is the next ROS address assuming that no interveninghardware interrupt occurs. Should a hardware interrupt occur, this newlygenerated nominal next ROS address will be placed on top of the returnaddress stack 70 and the generated hardware interrupt vector addressreplaces it as the next ROS address in RAR 63.

As in the case of test branch operation, the PUSH microcommand can beused along with major branch operations. Again, as indicated above,hardware interrupts must be inhibited by setting bit RDDT34 to a binaryONE. If a PUSH microcommand is used in conjunction with a major branchoperation, the current ROS address which is the address of the majorbranch microinstruction itself (from ROS address history register 66)plus 1 will be placed on top of the return address stack 70 and themicroprogram will branch to the nominal next address as determined bythe output of major branch logic 57.

The incrementing with a constant operation (INCK microcommand) asspecified in ROS address field (bit 0 through 3 of the 13-bit field) ofthe current microinstruction causes the current value of the ROS addresshistory register 66 incremented by 1 to be placed in the ROS addressregister for the next microcycle. In addition to this next addressgeneration, the remaining 9 bits (RDDT bits 4 through 12) are used togenerate an 8-bit constant plus a filler to the 20-bit wide internal bus38 during the current microcycle.

The current ROS address contained in ROS address history register 66 isincremented by incrementer 64 and the result is returned to the ROSaddress register 63 via address multiplexer 1 60 and address multiplexer2 62 when an increment with constant microcommand is specified in theROS address field of a microinstruction. Should a hardware interruptoccur, this newly generated next address will be placed on the top ofreturn address stack 70 via return multiplexer 61 and the hardwaregenerated interrupt vector address will be placed in ROS addressregister 63 to be used as the next ROS address. The PUSH subcommand canbe used along with the increment operations. As with all PUSHsubcommands, the hardware interrupts must be inhibited by setting RDDTbit 34 to a binary ONE when using a PUSH subcommand in conjunction withan increment operation. If a PUSH microcommand is used in conjunctionwith an increment operation, the current ROS address incremented by onewill be placed on the top of return address stack 70, in addition tobecoming the next ROS address.

The increment operation (INC) subcommand as specified in the ROS addressfield of the microinstruction initiates the same operation as describedabove for the increment with constant (INCK microcommand) operationexcept that no constant is generated onto internal bus 38.

The return operation (POP microcommand) causes the contents of the topof return address stack 70 to be loaded into ROS address register 63 viaaddress multiplexer 1 60 and address multiplexer 2 62 to be used as theROS address for the next microcycle. In addition, a ROS address of 001(hexadecimal) is loaded into the bottom of the stack into register 74each time return address stack 70 is popped one location. This loadingof the bottom of return address stack 70 with the ROS address of 1 isused to detect the case of overpopping of the stack. The overpopping ofthe stack will result in the microprocessor being vectored tomicroprogram error sequence which begins at ROS location 1. Because areturn operation (POP microcommand) is fully specified by bit 0 through4 of the ROS address field, bits 5 through 12 of the ROS address fieldare unused as part of the return operation.

When a return operation is specified in bit 0 through 4 of the ROSaddress field, ROS address register 63 receives the contents of the topof return address stack assuming no intervening hardware interruptoccurs. Should a hardware interrupt occur, the return (or pop) operationwill effectively be bypassed or cancelled. This cancelling of the popstack operation when a hardware interrupt occurs during a returnoperation is the logical equivalent of the popping the return addressfrom the top of return address stack 70 and immediately, within the samemicrocycle, pushing it back onto the top of return address stack 70. Itis this cancelling or bypassing of popping return address stack 70 whena hardware interrupt occurs during a return operation that allows thereturn address stack to be used to contain the return addresses for bothmicroprogram subroutine calls and for hardware interrupts.

By having the push operation onto the stack associated with storing thereturn address for the hardware interrupt routine cancel the popoperation performed on the stack when returning from a microprogramsubroutine or upon completion of a hardware interrupt service routine,the return address stack 70 does not have to be able to simultaneouslymove in opposite directions or to first pop up and then push down duringone microcycle. This cancelling of the stack pop operation associatedwith a return operation by the occurrence of the push operationassociated with the occurrence of a hardware interrupt does notadversely affect the flow of control within the microprogram because thehardware interrupt routine will perform a return operation as the laststep in its microprogrammed interrupt service routine.

The PUSH microcommand which is coded in the special control field (RDDTbits 35 through 47) must not be used in the same microinstruction with areturn operation which is coded within the ROS address field (RDDT bit 0through 12) because the results within microprocessor 30 in thepreferred embodiments are unspecified.

As described above, a hardware interrupt forces a branch to a fixed ROSaddress. This ROS address is determined by a priority network (prinet59) which has various error signals and interrupt requests as inputsfrom register 58. Hardware interrupts cause the next firmware generatedROS address to be pushed onto the top of return address stack 70. If thenext ROS address was to have been generated from the return addressstack via a return microcommand, popping of the return address stack 70is inhibited. Hardware interrupts must be inhibited whenever a PUSHmicro-operation is performed in order to prevent the requirement todoubly push the stack, the first push being associated with the PUSHmicrocommand itself and the second push being associated with the savingof the return address for the hardware interrupt.

When the hardware interrupt inhibit field (RDDT bit 34) is a binary ONE,hardware interrupts are inhibited, non-error condition hardwareinterrupts (such as those associated with memory refresh and datarequest) are inhibited (prevented from intervening between the executionof the current microinstruction and the execution of the nextmicroinstruction). Hardware error condition inputs to prinet 59 (such assystem clear, an attempt to access a nonexistent resource, an accessviolation, or a memory parity error) are not under the control of RDDTbit 34 and can intervene between any two given microcycles. The factthat hardware interrupts associated with these error conditions can notbe inhibited and therefore could occur during a microinstruction whichcontains a PUSH microcommand does not cause a problem, because thehardware interrupt service routines associated with these noninhibitableerror conditions does not do a return operation at the end and thereforedo not depend upon the contents of return address stack 70 to be valid.When RDDT bit 34 is a binary ZERO, all hardware interrupts are allowed.

COMMERCIAL INSTRUCTION LOGIC OVERVIEW

Referring now to FIG. 4 which illustrates the commercial instructionlogic 28, of FIG. 1 in greater detail. FIG. 4 also illustratesmicroprocessor 30, read-only storage 24 and monitor logic 22.

In FIG. 4, the number next to the upper right-hand corner of the blocksrepresents the number of bits of information contained in the registerrepresented by the block and the numbers within parentheses next tosignal lines represents the number of parallel signals transmitted overthe signal path.

As indicated above, commercial instruction logic 28 is used inconjunction with microprocessor 30 to perform the commercial softwareinstructions of CPU 20 which do decimal arithmetic operations on decimaldata and editing operations on alphanumeric data strings. Commercialinstruction logic 28 consists primarily of random access memory (RAM) 181, RAM 2 96 and decimal adder/subtractor PROM 84, all of which operateunder the control of CIL control area 100. As will be described ingreater detail below, CIL control area 100 is used to decode the bitswithin the firmware microinstruction word which control commercialinstruction logic 28. In particular, CIL control area receives bits 35through 47 of the microinstruction firmware word shown as the specialcontrol field in FIG. 5, which are also used to control the operation ofmicroprocessor 30 and in addition it receives bits 48 through 55 whichare dedicated to the control of commercial instruction logic 28.Decoding of these microinstruction bits associated with the commercialinstruction logic is performed by CIL control area 100 which producescontrol signals which are distributed throughout commercial instructionlogic 28 to control the enabling and disabling and the selection of thevarious registers, gates and multiplexers.

Data is transmitted between microprocessor 30 and commercial instructionlogic 28 over a 16-bit wide data path which connects processor bus 37 ofmicroprocessor 30 to transceivers 97. The output of transceivers 97 canbe latched into data-in register 98 wich is also 16 bits wide.

As can be seen in FIG. 4, transceivers 97 can not only load data-inregister 98 from the output of processor bus 37, but can also be used totransfer the output of RAM 2 data register 88 from transceivers 97 toprocessor bus 37. This data path from the output of RAM 2 data register88 which is 16 bits wide, besides being connected into transceivers 97can also be used to load data-in register 98 and thereby provide themeans for loading the output of RAM 2 96 into RAM 1 81 or back into RAM2 96. When commercial instruction logic 28 is used to perform acommercial software instruction which requires two operands, operand 1is usually stored in RAM 1 81 and operand 2 is stored in RAM 2 96 andthe output of the operation is stored back into RAM 2 96.

As indicated, the arithmetic and logic unit functions of commercialinstruction logic 28 are performed by decimal adder/subtractor PROM 84.PROM 84 generates the result by using its four inputs (2 4-bit operands,1 bit of carry-in and a 1-bit indicator indicating whether this is anadd or subtract operation) to form a 10-bit address which is used tofetch an 8-bit data word which contains a 4-bit arithmetic result of theaddition or subtraction and four indicators (one bit of carry-out, onebit to indicate whether one of the operands is an illegal value, a bitindicating whether the result is equal to zero, and a bit indicatingwhether the result is equal to nine). The coding of decimaladder/subtractor PROM 84 is shown in Table 3.

Table 3 shows the encoding of the 10-bit address. The most significantbit in the 10-bit address, which has a value of 512, is used as anindicator as to whether the operation being performed is an addition orsubtraction. Thus, when address bit 512 is equal to a binary ZERO, asubtraction is to be performed, and when equal to a binary ONE, anaddition is to be performed. The next address bit, having a value of256, is used to indicate whether a carry in from the previous decimaldigit is to be used in calculating the result and when a binary ZEROindicates that the previous digit did not generate a carry-out and whena binary ONE indicates that the previous digit did generate a carry-out.The next four address bits, bits having a value of 128, 64, 32 and 16,are used to represent the four bits of operand 2 at the B port ofdecimal ALU 84 and the last four bits having values of 8, 4, 2 and 1 areused to indicate the values of operand 1 at the A port of decimal ALU84. The 8-bit data word retrieved form PROM 1 as addressed by the 10-bitaddress is coded with the result as indicated in Table 3. The four lowerbits of the data word (bits 3 through 0) contain the 4-bit decimalresult of the addition or subtraction. The other four bits of the dataword contain the four indicators which are output by decimaladder/subtractor PROM 84.

The 4-bit indicators are encoded such that bit 7 of the data word(labeled "CRO" in Table 3), when a binary ZERO, indicates that there isno carry-out and, when a binary ONE, indicates that there was acarry-out. Bit 6 (labeled "ILL" in Table 3), when a binary ZERO,indicates that both operand 1 and operand 2 were legal decimal valuesand when a binary ONE, indicates that one or the other of operand 1 oroperand 2 contained an illegal decimal value (i.e., a hexadecimal valueof from A through F), bit 5 (labeled "E0" in Table 3) indicates when abinary ONE that the arithmetic result is equal to zero and when a binaryZERO that the arithmetic result is not equal to zero, and bit 4 (labeled"E9" in Table 3) when a binary ONE indicates that the arithmetic resultequal nine and when a binary ZERO indicates that the arithmetic resultdoes not equal nine. As can be seen in FIG. 4, the four indicator bitsare held by decimal indicators 85 and are also an input into monitormultiplexer 80 and the carry-out bit is input into decimaladder/subtractor 84 as the carry-in bit. The 4-bit decimal arithmeticresult is one input into result/zone multiplexer 91.

                                      TABLE 3                                     __________________________________________________________________________    Decimal Add/Subtract PROM Encoding                                                                8 DATA BITS                                               10 ADDRESS BITS.    3 2 1 0                                                                              7  6  5 4                                          512 256 128 64 32 16                                                                         8 4 2 1                                                                            DECIMAL                                                                              INDICATORS                                         ADD CRI OP2    OP1  RESULT CRO                                                                              ILL                                                                              E0                                                                              E9                                         __________________________________________________________________________    0   0   0      0    0      0  0  1 0                                          0   0   0      1    9      1  0  0 1                                          0   0   0      2-9  8-1    1  0  0 0                                          0   0   0      A-F  A-F    0  1  0 0                                          0   0   1      0    1      0  0  0 0                                          0   0   1      1    0      0  0  1 0                                          0   0   1      2    9      1  0  0 1                                          0   0   1      3-9  8-2    1  0  0 0                                          0   0   1      A-F  A-F    0  1  0 0                                          0   0   2      0-1  2-1    0  0  0 0                                          0   0   2      2    0      0  0  1 0                                          0   0   2      3    9      1  0  0 1                                          0   0   2      4-9  8-3    1  0  0 0                                          0   0   2      A-F  A-F    0  1  0 0                                          0   0   3      0-2  3-1    0  0  0 0                                          0   0   3      3    0      0  0  1 0                                          0   0   3      4    9      1  0  0 1                                          0   0   3      5-9  8-4    1  0  0 0                                          0   0   3      A-F  A-F    0  1  0 0                                          0   0   4      0-3  4-1    0  0  0 0                                          0   0   4      4    0      0  0  1 0                                          0   0   4      5    9      1  0  0 1                                          0   0   4      6-9  8-5    1  0  0 0                                          0   0   4      A-F  A-F    0  1  0 0                                          0   0   5      0-4  5-1    0  0  0 0                                          0   0   5      5    0      0  0  1 0                                          0   0   5      6    9      1  0  0 1                                          0   0   5      7-9  8-6    1  0  0 0                                          0   0   5      A-F  A-F    0  1  0 0                                          0   0   6      0-5  6-1    0  0  0 0                                          0   0   6      6    0      0  0  1 0                                          0   0   6      7    9      1  0  0 1                                          0   0   6      8-9  8-7    1  0  0 0                                          0   0   6      A-F  A-F    0  1  0 0                                          0   0   7      0-6  7-1    0  0  0 0                                          0   0   7      7    0      0  0  1 0                                          0   0   7      8    9      1  0  0 1                                          0   0   7      9    8      1  0  0 0                                          0   0   7      A-F  A-F    0  1  0 0                                          0   0   8      0-7  8-1    0  0  0 0                                          0   0   8      8    0      0  0  1 0                                          0   0   8      9    9      1  0  0 1                                          0   0   8      A-F  A-F    0  1  0 0                                          0   0   9      0    9      0  0  0 1                                          0   0   9      1-8  8-1    0  0  0 0                                          0   0   9      9    0      0  0  1 0                                          0   0   9      A-F  A-F    0  1  0 0                                          0   0   A      0-9  A      0  1  0 0                                          0   0   A      A    A      0  1  1 0                                          0   0   A      B-F  A      0  1  0 0                                          0   0   B      0-A  B      0  1  0 0                                          0   0   B      B    B      0  1  1 0                                          0   0   B      C-F  B      0  1  0 0                                          0   0   C      0-B  C      0  1  0 0                                          0   0   C      C    C      0  1  1 0                                          0   0   C      D-F  C      0  1  0 0                                          0   0   D      0-C  D      0  1  0 0                                          0   0   D      D    D      0  1  1 0                                          0   0   D      E-F  D      0  1  0 0                                          0   0   E      0-D  E      0  1  0 0                                          0   0   E      E    E      0  1  1 0                                          0   0   E      F    E      0  1  0 0                                          0   0   F      0-E  F      0  1  0 0                                          0   0   F      F    F      0  1  1 0                                          0   1   0      0    9      1  0  0 1                                          0   1   0      1-8  8-1    1  0  0 0                                          0   1   0      9    0      1  0  1 0                                          0   1   0      A-F  A-F    0  1  0 0                                          0   1   1      0    0      0  0  1 0                                          0   1   1      1    9      1  0  0 1                                          0   1   1      2-9  8-1    1  0  0 0                                          0   1   1      A-F  A-F    0  1  0 0                                          0   1   2      0    1      0  0  0 0                                          0   1   2      1    0      0  0  1 0                                          0   1   2      2    9      1  0  0 1                                          0   1   2      3-9  8-2    1  0  0 0                                          0   1   2      A-F  A-F    0  1  0 0                                          0   1   3      0-1  2-1    0  0  0 0                                          0   1   3      2    0      0  0  1 0                                          0   1   3      3    9      1  0  0 1                                          0   1   3      4-9  8-3    1  0  0 0                                          0   1   3      A-F  A-F    0  1  0 0                                          0   1   4      0-2  3-1    0  0  0 0                                          0   1   4      3    0      0  0  1 0                                          0   1   4      4    9      1  0  0 1                                          0   1   4      5-9  8-4    1  0  0 0                                          0   1   4      A-F  A-F    0  1  0 0                                          0   1   5      0-3  4-1    0  0  0 0                                          0   1   5      4    0      0  0  1 0                                          0   1   5      5    9      1  0  0 1                                          0   1   5      6-9  8-5    1  0  0 0                                          0   1   5      A-F  A-F    0  1  0 0                                          0   1   6      0-4  5-1    0  0  0 0                                          0   1   6      5    0      0  0  1 0                                          0   1   6      6    9      1  0  0 1                                          0   1   6      7-9  8-6    1  0  0 0                                          0   1   6      A-F  A-F    0  1  0 0                                          0   1   7      0-5  6-1    0  0  0 0                                          0   1   7      6    0      0  0  1 0                                          0   1   7      7    9      1  0  0 1                                          0   1   7      8-9  8-7    1  0  0 0                                          0   1   7      A-F  A-F    0  1  0 0                                          0   1   8      0-6  7-1    0  0  0 0                                          0   1   8      7    0      0  0  1 0                                          0   1   8      8    9      1  0  0 1                                          0   1   8      9    8      1  0  0 0                                          0   1   8      A-F  A-F    0  1  0 0                                          0   1   9      0-7  8-1    0  0  0 0                                          0   1   9      8    0      0  0  1 0                                          0   1   9      9    9      1  0  0 1                                          0   1   9      A-F  A-F    0  1  0 0                                          0   1   A      0-9  A      0  1  0 0                                          0   1   A      A    A      0  1  1 0                                          0   1   A      B-F  A      0  1  0 0                                          0   1   B      0-A  B      0  1  0 0                                          0   1   B      B    B      0  1  1 0                                          0   1   B      C-F  B      0  1  0 0                                          0   1   C      0-B  C      0  1  0 0                                          0   1   C      C    C      0  1  1 0                                          0   1   C      D-F  C      0  1  0 0                                          0   1   D      0-C  D      0  1  0 0                                          0   1   D      D    D      0  1  1 0                                          0   1   D      E-F  D      0  1  0 0                                          0   1   E      0-D  E      0  1  0 0                                          0   1   E      E    E      0  1  1 0                                          0   1   E      F    E      0  1  0 0                                          0   1   F      0-E  F      0  1  0 0                                          0   1   F      F    F      0  1  1 0                                          1   0   0      0    0      0  0  1 0                                          1   0   0      1-9  1-9    0  0  0 0                                          1   0   0      A-F  A-F    0  1  0 0                                          1   0   1      0-8  1-9    0  0  0 0                                          1   0   1      9    0      1  0  1 0                                          1   0   1      A-F  A-F    0  1  0 0                                          1   0   2      0-7  2-9    0  0  0 0                                          1   0   2      8    0      1  0  1 0                                          1   0   2      9    1      1  0  0 0                                          1   0   2      A-F  A-F    0  1  0 0                                          1   0   3      0-6  3-9    0  0  0 0                                          1   0   3      7    0      1  0  1 0                                          1   0   3      8-9  1-2    1  0  0 0                                          1   0   3      A-F  A-F    0  1  0 0                                          1   0   4      0-5  4-9    0  0  0 0                                          1   0   4      6    0      1  0  1 0                                          1   0   4      7-9  1-3    1  0  0 0                                          1   0   4      A-F  A-F    0  1  0 0                                          1   0   5      0-4  5-9    0  0  0 0                                          1   0   5      5    0      1  0  1 0                                          1   0   5      6-9  1-4    1  0  0 0                                          1   0   5      A-F  A-F    0  1  0 0                                          1   0   6      0-3  6-9    0  0  0 0                                          1   0   6      4    0      1  0  1 0                                          1   0   6      5-9  1-5    1  0  0 0                                          1   0   6      A-F  A-F    0  1  0 0                                          1   0   7      0-2  7-9    0  0  0 0                                          1   0   7      3    0      1  0  1 0                                          1   0   7      4-9  1-6    1  0  0 0                                          1   0   7      A-F  A-F    0  1  0 0                                          1   0   8      0-1  8-9    0  0  0 0                                          1   0   8      2    0      1  0  1 0                                          1   0   8      3-9  1-7    1  0  0 0                                          1   0   8      A-F  A-F    0  1  0 0                                          1   0   9      0    9      0  0  0 0                                          1   0   9      1    0      1  0  1 0                                          1   0   9      2-9  1-8    1  0  0 0                                          1   0   9      A-F  A-F    0  1  0 0                                          1   0   A      0-9  A      0  1  0 0                                          1   0   A      A    A      0  1  1 0                                          1   0   A      B-F  A      0  1  0 0                                          1   0   B      0-A  B      0  1  0 0                                          1   0   B      B    B      0  1  1 0                                          1   0   B      C-F  B      0  1  0 0                                          1   0   C      0-B  C      0  1  0 0                                          1   0   C      C    C      0  1  1 0                                          1   0   C      D-F  C      0  1  0 0                                          1   0   D      0-C  D      0  1  0 0                                          1   0   D      D    D      0  1  1 0                                          1   0   D      E-F  D      0  1  0 0                                          1   0   E      0-D  E      0  1  0 0                                          1   0   E      E    E      0  1  1 0                                          1   0   E      F    E      0  1  0 0                                          1   0   F      0-E  F      0  1  0 0                                          1   0   F      F    F      0  1  1 0                                          1   1   0      0-8  1-9    0  0  0 0                                          1   1   0      9    0      1  0  1 0                                          1   1   0      A-F  A-F    0  1  0 0                                          1   1   1      0-7  2-9    0  0  0 0                                          1   1   1      8    0      1  0  1 0                                          1   1   1      9    1      1  0  0 0                                          1   1   1      A-F  A-F    0  1  0 0                                          1   1   2      0-6  3-9    0  0  0 0                                          1   1   2      7    0      1  0  1 0                                          1   1   2      8-9  1      1  0  0 0                                          1   1   2      A-F  A-F    0  1  0 0                                          1   1   3      0-5  4-9    0  0  0 0                                          1   1   3      6    0      1  0  1 0                                          1   1   3      7-9  1-3    1  0  0 0                                          1   1   3      A-F  A-F    0  1  0 0                                          1   1   4      0-4  5-9    0  0  0 0                                          1   1   4      5    0      1  0  1 0                                          1   1   4      6-9  1-4    1  0  0 0                                          1   1   4      A-F  A-F    0  1  0 0                                          1   1   5      0-3  6-9    0  0  0 0                                          1   1   5      4    4      1  0  1 0                                          1   1   5      5-9  1-5    1  0  0 0                                          1   1   5      A-F  A-F    0  1  0 0                                          1   1   6      0-2  7-9    0  0  0 0                                          1   1   6      3    0      1  0  1 0                                          1   1   6      4-9  1-6    1  0  0 0                                          1   1   6      A-F  A-F    0  1  0 0                                          1   1   7      0-1  8-9    0  0  0 0                                          1   1   7      2    0      1  0  1 0                                          1   1   7      3-9  1-7    1  0  0 0                                          1   1   7      A-F  A-F    0  1  0 0                                          1   1   8      0    9      0  0  0 0                                          1   1   8      1    0      1  0  1 0                                          1   1   8      2-9  1-8    1  0  0 0                                          1   1   8      A-F  A-F    0  1  0 0                                          1   1   9      0    0      1  0  1 0                                          1   1   9      1-9  1-9    1  0  0 0                                          1   1   9      A-F  A-F    0  1  0 0                                          1   1   A      0-9  A      0  1  0 0                                          1   1   A      A    A      0  1  1 0                                          1   1   A      B-F  A      0  1  0 0                                          1   1   B      0-A  B      0  1  0 0                                          1   1   B      B    B      0  1  1 0                                          1   1   B      C-F  8      0  1  0 0                                          1   1   C      0-B  C      0  1  0 0                                          1   1   C      C    C      0  1  1 0                                          1   1   C      D-F  C      0  1  0 0                                          1   1   D      0-C  D      0  1  0 0                                          1   1   D      D    D      0  1  1 0                                          1   1   D      E-F  D      0  1  0 0                                          1   1   E      0-D  E      0  1  0 0                                          1   1   E      E    E      0  1  1 0                                          1   1   F      F    E      0  1  0 0                                          1   1   F      0-E  F      0  1  0 0                                          1   1   F      F    F      0  1  1 0                                          __________________________________________________________________________

The ability to detect whether either of the two operands contains avalue greater than 9 is very useful in that it allows the decimaladder/subtractor PROM 84 to detect the case of an illegal operand whichhas a decimal digit which is represented by four bits and therefore canhave values of from A through F hexadecimal which are illegal. Theability for the decimal adder/subtractor PROM 84 to detect illegaldecimal operands as part of the addition/subtraction process eliminatesthe need for a separate precheck of the operands by a prescan of theoperands prior to introducing them into the ALU.

Sign detector PROM 78 is similar in operation to decimaladder/subtractor PROM 84. Sign detector PROM 78 uses the nine input bitsto address a 4-bit data word which indicates the sign of the operandsused in an arithmetic operation. Of the nine bits used to address the4-bit data words of the sign detector PROM 78, three bits come fromdata-in register 98, 4 bits from sign multipliexer 77, and two bits(PACKD) and overpunch (OVPUN) from CIL control area 100. The output ofsign detector PROM 78 can be gated to monitor logic 22 for inputing intomicroprocessor 30 via monitor multiplexer 80. The four bits output bysign detector PROM 78 indicate whether the sign is positive or negative,whether it is an illegal sign atom, whether the sign is an overpunchsign, and whether it is an overpunched zero. The resultant sign can betested by the microprocessor 30 firmware examining the four monitor bitsof test flip-flops 40 (see FIG. 3). In the preferred embodiment, thegeneration of the sign result is complex in that the CPU 20 allowsdecimal numbers to be represented in either a packed or unpacked formatwith trailing or leading signs and overpunch signs.

RAM 1 zero multiplexer 82 at the output of RAM 1 81 and RAM 2 zeromultiplexer 90 at the output of RAM 2 96 are used to allow thecommercial instruction logic firmware to effectively zero out the outputof RAM 1 and RAM 2 respectively so that the operand in the other RAM caneffectively be added or subtracted from zero.

RAM 2 data register 88 holds the 16-bit output of RAM 2 96. RAM 2 nibblemultiplexer 89 is used to select the one of the four 4-bit nibbles heldin RAM 2 data register 88 so that the appropriate nibble can be gatedinto RAM 2 zero multiplexer 90 or into double multiplexer 83 inpreparation of adding or subtracting a nibble from RAM 2 96 with anibble from RAM 1 81 in decimal adder/subtractor PROM 84.

Nibble 0 multiplexer 92, nibble 1 multiplexer 93, nibble 2 multiplexer94, and nibble 3 multiplexer 95 are used to either allow a 16-bitquantity to be loaded in from data-in register 98 into RAM 2 96 or toallow a 4-bit nibble from result/zone multiplexer 91 into theappropriate 4-bit nibble of the 16-bit word stored in RAM 2 96 underfirmware control.

Result/zone multiplexer 91 is used to determine whether the 4-bit resultfrom decimal adder/subtractor PROM 84 or the four zone bits are to bewritten into a nibble within RAM 2 96. In the CPU of the preferredembodiment, when a decimal number is stored in a packed format, eachfour bits of data in the decimal value represent a decimal digit havingthe value from zero through nine. When decimal data is stored in anunpacked (string) format, each decimal digit is represented by an eightbits in which the most significant (left) four bits of the 8-bit fieldrepresent a zone field having a value of 3 hexadecimal and the leastsignificant (right) four bits represent the decimal values 0 through 9.Thus, in the unpacked format, each decimal digit is represented by an8-bit field which contains the ASCII code for the decimal digit.Therefore, result/zone multiplexer 91 can select between the 4-bitresult from decimal adder/subtractor PROM 84 (which can have a valuefrom 0 through 9) or the four zone bits which are preset to 0011 binary(3 hexadecimal). For example, the decimal value 76 when stored in apacked field is stored in eight bits with the most significant 4-bitnibble containing the value 7 and the least significant 4-bit nibblecontaining the value 6. When the same value of 76 decimal is stored inan unpacked field, it is stored in two 8-bit bytes with the left most4-bit nibble of each byte being a 4-bit zone field which contains thehexadecimal value of three and the right 4-bit nibble in each bytecontaining the decimal value of 7 in the left byte and 6 in the rightbyte. Therefore, the value of 76 decimal in a packed decimal field isrepresented by the 8-bit field of 76 hexadecimal and in an unpackedfield it is represented by a 16-bit field containing 3736 hexadecimal.

Double multiplexer 83 allows one input of decimal adder/subtractor PROM84 to be selected between four bits from RAM 1 81 or four bits from RAM2 96. When double multiplexer 83 selects one input of decimaladder/subtractor PROM 84 to be four bits from RAM 2 96, the effect isthat the output of decimal adder/subtractor PROM 84 is double the valueof the four bits from RAM 2 because in this case both inputs to thedecimal adder/subtractor PROM 84 will be from RAM 2. This provides avery convenient method of multiplying by two the operand stored in RAM 296.

Although both RAM 1 81 and RAM 2 96 contain 16-bit data words, with RAM1 containing 16 such 16-bit words and RAM 2 containing 256 16-bit words,there are other distinctions between these two RAMs. RAM 1, as indicatedabove, normally holds operand 1 and may be written into only on a 16-bitword basis. That is, when information is written into RAM 1 from data-inregister 98, all four nibbles of the 16-bit word are written to. Theoutput of RAM 1 is always a single 4-bit nibble which is presented atone of two inputs to RAM 1 zero multiplexer 82. The word which is readfrom RAM 1 81 is controlled by RAM 1 address counter 75 and the nibblewhich is enabled into one input of RAM 1 zero multiplexer 82 iscontrolled by nibble out control 76. RAM 1 address counter 75 alsoreceives inputs from nibble out control 76 such that consecutive decimaldigits may be accessed from RAM 1 by either incrementing or decrementinga nibble counter in nibble out control 76 which in turn eitherincrements or decrements the word counter in RAM 1 address counter 75each time that four nibbles have been accessed.

RAM 2, as indicated above, normally holds operand 2. RAM 2 96 has theability to be written into as one 16bit word or the ability to write anyone of the four individual nibbles. The 16-bit word to be written intoRAM 2 96 comes from data-in register 98 which can be loaded fromprocessor bus 37 or from the output of RAM 2 data register 88.Individual nibbles are written into RAM 2 96 from the output ofresult/zone multiplexer 91 with the value of the nibble being either theresult of the decimal adder/subtractor PROM 84 or a 4-bit zone fieldcontaining the hexadecimal value of 3. Nibble write control 86determines whether a 16-bit word is written into RAM 2 96 or whether oneof four individual nibbles is written into RAM 2 96. RAM 2 addresscounter 87 determines the 8-bit word address that is used to address RAM2 96 for either a read or write operation. RAM 2 address counter 87receives an input from nibble write control 86 so that consecutivenibbles may be accessed from RAM 2 96. Each time the four nibbles havebeen accessed, RAM 2 address counter 87 is either incremented ordecremented to get to the next word which contains the next consecutivenibble. A read from RAM 2 96 results in a 16-bit data word being readout and latched into RAM 2 data register 88. The particular nibble to beaccessed in the 16-bit data word is controlled by RAM 2 nibblemultiplexer 89 which selects one of the four nibbles to be output intoRAM 2 zero multiplexer 90 and double multiplexer 83. By contrast, a readin RAM 1 results in only four bits of one nibble being output with theother twelve bits of the three nibbles not being enabled. The enablingof the nibble which is read from RAM 1 81 is controlled by nibble outcontrol 76. The 16-bit word read from RAM 2 96 into RAM 2 data register88 can be written back into RAM 2 96 via data-in register 98 or it canbe written into RAM 1 81 via data-in register 98 under firmware control.The output of nibble write control 86, besides controlling which one offour nibbles is write enabled into RAM 2 when a single nibble is beingwritten into, also controls the selection of which one of four nibbleswill be output by RAM 2 nibble multiplexer 89. When a 16-bit word iswritten into RAM 2 96, all four nibbles are write enabled by nibblewrite control 86.

The sizes of RAM 1 81 and RAM 2 96 are determined by their use by thefirmware. As indicated above, RAM 1 81 is used primarily to hold operand1 which, in the CPU of the preferred embodiment, may be a decimal numberof from 1 through 31 decimal digits including the sign. Therefore, toaccommodate a 31 digit decimal number, sixteen words of 16 bits each arerequired in order to be able to hold the 31 bytes of the maximum decimalnumber if the number is an unpacked decimal number having zone bitsassociated with each decimal digit. RAM 2 96, on the other hand, besidesnormally holding operand 2 is divided into eight segments with eachsegment being used to either hold an operand or as a working register.For example, in the case of a decimal divide, the segments of RAM 2 96are used to hold an original copy of operand 1, an original copy ofoperand 2, a packed copy of operand 2, the quotient and partialproducts.

COMMERCIAL INSTRUCTION LOGIC MICROOPERATIONS

The microoperations of the microinstruction firmware word which controlthe operation of commercial instruction logic 28 will now be describedwith reference to FIG. 4 and FIG. 5 and Table 4 through Table 8. Asdiscussed earlier, the addressing of read-only storage 24 is controlledby microprocessor 30 and more particularly the control area 36 ofmicroprocessor 30. As shown in FIG. 3, the firmware microinstructionaccessed in ROS 24 is controlled by the contents of ROS address register63. Each time a firmware word is read from ROS 24 either 48 bits or 56bits are retrieved. If the address is within the first 2K of ROS (i.e.,addressed 0 through 2047) a 48-bit microinstruction word is retrievedand if the address is from 2048 through 4095 a 56-bit microinstructionword is retrieved. As described earlier, bits 0 through 47 ofmicroinstruction word are always latched into ROS data register 65 fordecoding and controlling the operations of microprocessor 30 (see FIG.3). In addition, the special control field of the microinstruction word,bits 35 through 47, are also latched into ROS SCF register 101 of CILcontrol area 100 (see FIG. 4). This allows the special control field,bits RDDT35 through RDDT47, to control either operations withinmicroprocesor 30 or within commercial instruction logic 28. As will beseen below in describing the microoperations in Table 4 and Table 5,subfield A of the special control field is used to enable some of themicrooperations controlled by subfields B and C. That is, as is shown inTable 4, three of the four microoperations defined by subfield B of thespecial control field are enabled only when subfield A contains a binary110 in RDDT35 through RDDT37. Similarly, the eight microoperationsdefined in Table 5 by the subfield C are enabled only when subfield Acontains a binary 110 in RDDT35 through RDDT37. This is shown in FIG. 4which shows that three of the bits from subfield B are enabled byfeeding the three bits from subfield A into AND gate 107 and that thedecoding of subfield C is enabled by feeding the three bits fromsubfield A into the enable (EN) input of decoder 106. As is shown inTable 6, and in FIG. 4, the three bits from subfield D do not requirethat subfield A be equal to a binary 110 in bits RDDT35 through RDDT37.The requirement that certain of the microoperations specified insubfield B, C and D of the special control field are enabled only ifsubfield A contains a binary 110 is necessary in order to inhibitcommercial instruction logic 28 from performing certain microoperationswhich would otherwise interfere with the microoperations being performedby microprocessor 30.

Zero multiplexer 102 is used to force binary ZEROs into ROS CIL register103 when the microprocessor 30 addresses a firmware location in thelower 2K of ROS 24. This forcing of binary ZEROs into ROS CIL register103 is done by enabling the tri-state outputs of zero register 102 anddisabling the tri-state outputs of ROS 24 bits RDDT48 through RDDT55. Byforcing ZEROs into ROS CIL register 103 in this manner, decode PROM 104and decode PROM 105 decode no operation microoperations (see Table 7 andTable 8) thereby inhibiting commercial instruction logic 28 fromperforming any operation when a firmware word has been retireved fromROS 24 which does not contain bits RDDT48 through RDDT55 (see FIG. 5).Therefore, zero multiplexer 102 is used to force zeros into subfields Eand F which are dedicated to commercial instruction logic 28 and whichare absent in the first 2K words in ROS 24. The sixteen microoperationsencoded within subfield E are shown in Table 7 and the sixteenmicrooperations specifiable in subfield F are shown in Table 8.

As can be appreciated by examining FIG. 4 and Table 4 through Table 8,many parallel microoperations for control of commercial instructionlogic can be programmed into a single microinstruction word. Becausesubfield B which contains four bits is not encoded, four parallelmicrooperations can be programmed into subfield B. The subfield Boperations can be performed in parallel with any of the sevenmicrooperations that can be programmed into the three bits which areencoded in subfield C (see Table 5). Table 6 shows that three parallelmicrooperations can be programmed into subfield D and these in turn canbe performed in parallel with any of the operations of subfields B andC. Table 7 shows that this 4-bit field is encoded to provide one offifteen microoperations which can be performed in parallel with any ofthe operations of subfields B, C, D and F. Table 8 illustrates that thefour bits encoded to provide one of sixteen microoperations which can beperformed in parallel with any of the microoperations specified insubfield B, C, D and E.

It will be understood that the various control signals from CIL controlarea 100 are applied to the various logic elements of commercialinstruction logic 28 during each microinstruction execution cycle. Itwill also be understood that the clock signals from microprocessor 30provide appropriate timing signals in a conventional manner to thecommercial instruction logic 28 to provide appropriate timing therefor.In order not to confuse the drawings, the particular timing and controlsignals fed to various elements are not shown in FIGS. 1-4, but areassumed to be provided where appropriate.

The microoperations of Table 4 will now be described with reference toFIG. 4.

                  TABLE 4                                                         ______________________________________                                        Subfield B Microoperations                                                    RDDT Bits                                                                     35  36     37     38   39   40   41   Operation                               ______________________________________                                        X   X      X      1    X    X    X    Double RAM 2 thru                                                             Add/Sub PROM                                                                  (CIPDUB)                                1   1      0      X    1    0    X    Write RAM 2 Nibble                                                            (CWRESZ)                                1   1      0      X    X    1    X    Write RAM 2 All                                                               (CWROP2)                                1   1      0      X    X    X    1    Write RAM 1 All                                                               (CWROP1)                                ______________________________________                                    

In Table 4, it can be seen that when RDDT38 is a binary ONE, that bothinputs to decimal adder/subtractor PROM 84 originates from RAM 2 96which effectively allows, when an addition is being performed, thedoubling of the 4-bit nibble output from RAM 2. Thus, when RDDT38 is abinary ONE, the double multiplexer 83 selects as its output the input itreceived from RAM 2 nibble multiplexer 89. This also allows, byperforming a subtract operation in decimal adder/subtractor PROM 84 andby selecting the zero inputs as the output of RAM 2 zero multiplexer 90,the ability to subtract the nibble output from RAM 2 from zero and thusdo a complementing of the nibble from RAM 2. When RDDT39 is a binaryONE, one nibble is written into RAM 2 96. This is accomplished byenabling one of the four 4-bit segments of RAM 2 96 under the control ofnibble write control 86 so that only one nibble is written into the wordspecified by RAM 2 address counter 87.

RDDT40, when a binary ONE, controls multiplexers 92 through 95 such thatthe output of data-in register 98 is input into RAM 2 96 and all fournibbles in RAM 2 96 are write enabled such that a full word addressed byRAM 2 address counter 87 is written into RAM 2. When RDDT41 is a binaryONE, a full 16-bit word is written into RAM 1 81 from data-in register98 in the word specified by RAM 1 address counter 75. As can be seen inTable 4, in order for the microoperations controlled by RDDT39, RDDT40and RDDT41, subfield A of the special control field must have the binaryvalue 110 in RDDT35 through RDDT37. This can also be appreciated byexamining Table 4, the four microoperations controlled by RDDT38 throughRDDT41 can each be selected within a single microinstruction with theexception that in order to write a single nibble into RAM 2, RDDT40 mustbe a binary ZERO therefore preventing both the writing of a singlenibble and the writing of a full word into RAM 2 in a singlemicrooperation.

The microoperations controlled by subfield C of the special controlfield will now be discussed with reference to Table 5 and FIG. 4.

                  TABLE 5                                                         ______________________________________                                        Subfield C Microoperations                                                    RDDT Bits When RDDT35-37 Equals 110 Binary                                    RDDT Bits                                                                     42       43    44        Operation                                            ______________________________________                                        0        0     0         No Operation                                                                  (NOP)                                                0        0     1         Packed Sign                                                                   (CPACKD)                                             0        1     0         Overpunched Sign                                                              (COVPUN)                                             0        1     1         Subtract                                                                      (CIPSUB)                                             1        0     0         Reset =0 And =9 Indicators                                                    (CRSETX)                                             1        0     1         Increment Address RAM 1                                                       (CIAD01)                                             1        1     0         Decrement Address RAM 1                                                       (CDAD01)                                             1        1     1         Reset All Indicators                                                          (CRESET)                                             ______________________________________                                    

The three bits of subfield C, that is RDDT42 through RDDT44, are decodedby decoder 106 to perform one of eight microoperations. When RDDT42through RDDT44 are a binary 000, no operation is performed by commercialinstruction logic 28. When RDDT42 through RDDT44 are a binary 001, thePACKD signal input into sign detection PROM 78 is made a binary ZERO andis used to address the portion of the sign detector PROM 78 whichcontains the coding for packed decimal sign types. When RDDT42 throughRDDT44 are a binary 010, signal OVPUN becomes a binary ZERO and is usedto address the portion of the sign detector PROM 78 which contains theencoding for overpunched signs. When bits RDDT42 through RDDT44 are abinary 011, a subtract operation is performed by decimaladder/subtractor PROM 84 by forcing the signal ADD/SUB which is one ofits inputs to be a binary ZERO. When RDDT42 through RDDT44 are a binary100, the equal zero and equal nine indicators in decimal indicators 85are reset to zero. When RDDT42 through RDDT44 are a binary 101, theaddress counter in RAM 1 address counter 75 is incremented by one suchthat the next word in RAM 1 81 is addressed thereby allowing one of fournew nibbles to be input into RAM 1 zero multiplexer 82. The exact nibblewhich is enabled into RAM 1 zero multiplexer 82 is determined by nibbleout control 76. When RDDT42 through RDDT44 are a binary 110, RAM 1address counter 75 is decremented by one, thereby addressing the nextlower word in RAM 1 81 and making available at its output one of fournew nibbles with the enabled nibble again being under the control ofnibble output control 76. When RDDT42 through RDDT44 are a binary 111,all four indicators in decimal indicators 85 are reset to binary ZEROthereby indicating that there is no carry, that the result is not equalto zero, that the result is not equal to nine, and that the result isnot an illegal digit.

The microoperations associated with subfield D of special control fieldwill now be described with reference to Table 6.

                  TABLE 6                                                         ______________________________________                                        Subfield D Microoperations                                                    RDDT Bits                                                                     45     46    47       Operation                                               ______________________________________                                        1      X     X        Sign to Monitor Logic                                                         (CIPSGN)                                                X      1     X        Inhibit RAM 2 to Add/Sub PROM                                                 (CINOP2)                                                X      X     1        Inhibit RAM 1 to Add/Sub PROM                                                 (CINOP1)                                                ______________________________________                                    

As is shown in Table 6, each of these three bits is independent of theother three bits in subfield D and therefore all three microoperationsmay be performed in parallel. It should also be noted that themicrooperations controlled by subfield D do not require that thesubfield A be any particular value whereas when coding themicrooperations described above in Table 5, with respect to subfield C,require that subfield A contain a binary value of 110. When RDDT45 is abinary ONE, the output of sign detector PROM 78 is enabled throughmonitor multiplexer 80 to monitor logic 22. When RDDT45 is a binaryZERO, the output of monitor multiplexer 80 is the decimal indicators 85which are then transferred to monitor logic 22. When RDDT46 is a binaryONE, the zero input of RAM 2 zero multiplexer 90 is enabled to itsoutput and thus zeros are entered as one of the operands into thedecimal adder/subtractor PROM 84. When RDDT47 is a binary ONE, the zeroinput to RAM 1 zero multiplexer 82 is enabled into double multiplexer 83thereby inhibiting the output of RAM 1 from entering decimaladder/subtractor PROM 84.

The microoperations controlled by subfield E are illustrated in Table 7.As discussed above, these microoperations, as do those controlled bysubfield F shown in Table 8, are encoded in microinstruction bits 48through 55 which are only present in the upper 2K words of read onlystorage 24. Therefore, as indicated earlier, if the address used toaddress ROS 24 is less than 2K, the output of zero register 102 isenabled into ROS CIL register 103 thereby forcing bits RDDT48 throughRDDT55 to a binary ZERO which will cause two parallel no operations tobe performed by commercial instruction logic 28. When the ROS 24 addressis greater than 2K, the output of bits 48 through 55 of ROS 24 isenabled into ROS CIL register 103 and one of the microoperationsspecified in Table 7 and one of the microoperations specified in Table 8will be performed.

                  TABLE 7                                                         ______________________________________                                        Subfield E Microoperations                                                    RDDT Bits When ROS Address Greater Than 2K                                    RDDT Bits                                                                     48  49    50    51  Operation                                                 ______________________________________                                        0   0     0     0   No Operation                                                                  (NOP4)                                                    0   0     0     1   Load Address RAM 1                                                            (CLDAD1)                                                  0   0     1     0   Load Count RAM 1                                                              (CLDCT1)                                                  0   0     1     1   Load Address and Count RAM 1                                                  (CLDAC1) = (CLDAD1 & CLDCT1)*                             0   1     0     0   Load Address RAM 2                                                            (CLDAD2)                                                  0   1     0     1   Load Count RAM 2                                                              (CLDCT2)                                                  0   1     1     0   Load Address and Count RAM 2                                                  (CLDAC2) = (CLDAD2 & CLDCT2)*                             0   1     1     1   Load Address RAM 1 and RAM 2                                                  (CLDADB) = (CLDAD1 & CLDAD2)*                             1   0     0     0   Load Count RAM 1 and RAM 2                                                    (CLDCTB) = (CLDCT1 & CLDCT2)*                             1   0     0     1   Load Address and Count RAM 1 and RAM 2                                        (CLDACB) = (CLDAD1 & CLDCT1                                                   & CLDAD2 & CLDCT2)*                                       1   0     1     0   Count Up RAM 1                                                                (CTUCT1)                                                  1   0     1     1   Count Down RAM 1                                                              (CTDCT1)                                                  1   1     0     0   Count Up RAM 2                                                                (CTUCT2)                                                  1   1     0     1   Count Down RAM 2                                                              (CTDCT2)                                                  1   1     1     0   Count Up RAM 1 and RAM 2                                                      (CTUALL) = (CTUCT1 & CTUCT2)*                             1   1     1     1   Count Down RAM 1 and RAM 2                                                    (CTDALL) = (CTDCT1 & CTDCT2)*                             ______________________________________                                         *Parallel microoperations created by decode PROM coding.                 

Now referencing Table 7, when RDDT48 through RDDT51 are a binary 0000 ano operation is performed. When RDDT48 through RDDT51 are a binary 0001,an address from data-in register 98 is located into RAM 1 addresscounter 75 thereby permitting control of which word is addressed in RAM1 81. When RDDT48 through RDDT51 is a binary 0010, a nibble count fromdata-in register 98 is loaded into nibble output control 76 therebycontrolling which one of four nibbles contained in one word of RAM 1 isenabled into one input of RAM 1 zero multiplexer 82. When RDDT48 throughRDDT51 is a binary 0011, a word address is loaded into RAM 1 addresscounter 75 and a nibble count is loaded into nibble output control 76from data-in register 98 thereby allowing the specifying as to the wordand the nibble which will be read from RAM 1 81. The RDDT48 throughRDDT51 is a binary 0100, an initial address is loaded into RAM 2 addresscounter 87 from data-in register 98 thereby controlling which word willbe written into or read from RAM 2 96. When RDDT48 through RDDT51 is abinary 0101, an initial nibble count is loaded into nibble write control86 thereby controlling which nibble will be write enabled into RAM 2 96.The nibble counter within nibble write control 86 also controls which ofthe four nibbles from RAM 2 data register 88 is enabled onto the outputsof RAM 2 nibble multiplexer 89. Therefore, loading of the RAM 2 nibblecount by this microoperation controls both the write enabling into RAM 296 and the output enabling of RAM 2 nibble multiplexer 89. When RDDT48through RDDT51 is a binary 0110, the address counter in RAM 2 addresscounter 87 and the nibble counter in nibble write control 86 are loadedfrom data in register 98 thereby controlling the word that is addressedwithin RAM 2 and the nibble which is write enabled into RAM 2 and thenibble which is selected at the output of RAM 2 nibble multiplexer 89.The loading of the nibble counter by this microoperation only controlswhich nibble will be write enabled when a write is done and does notactually do a write into RAM 2.

When RDDT48 through RDDT51 are a binary 0111, the word address containedin data-in register 98 is loaded into RAM 1 address counter 75 and RAM 2address counter 87. When RDDT48 through RDDT51 is a binary 1000, thenibble count in data-in register 98 is loaded into the nibble outcontrol 76 and into nibble write control 86 thereby allowing theselection of one of four nibbles in RAM 1 and RAM 2. When RDDT48 throughRDDT51 is a binary 1001, the word address and the nibble count fromdata-in register 98 are loaded into RAM 1 address counter 75 and RAM 2address counter 87 and nibble out control 76 and nibble write control 86respectively. When RDDT48 through RDDT51 is a binary 1010, the nibblecounter in nibble out control 76 is incremented by one and if it changesfrom a nibble count of three to a nibble count of zero, the word counterin RAM 1 address counter 75 is also incremented by one. This allowsnibbles to be consecutively addressed and after the four nibbles havebeen addressed from one word, the first nibble is addressed in the nextword. When RDDT48 through RDDT51 is a binary 1011, the nibble counter innibble out control 76 is decremented by one and if the count changesfrom zero to three, the word counter in RAM 1 address counter 75 isdecremented by one thereby allowing consecutive nibbles to be addressedfrom right to left.

When RDDT48 through RDDT51 is a binary 1100, the nibble counter in writecontrol 86 is incremented by one and if the nibble counter goes fromthree to zero, the word counter in RAM 2 address counter 87 is alsoincremented by one thereby allowing consecutive nibbles to be addressedfrom left to right. When RDDT48 through RDDT51 is a binary 1101, thenibble counter in nibble write control 86 is decremented by one and ifthe count goes from zero to three, the word counter in RAM 2 addresscounter 87 is also decremented by one to point to the next word in RAM 296. This decrementing of the nibble counter associated with RAM 2 by oneallows consecutive nibbles in RAM 2 to be addressed from right to left.When RDDT48 through RDDT51 is a binary 1110, the nibble counter innibble out control 76 is incremented by one as is the nibble counter innibble write control 86 and when a counter go from three to zero, thecorresponding word counter in RAM 1 address counter 75 and RAM 2 addresscounter 87 is also incremented by one thereby allowing consecutivenibbles to be addressed from left to right in RAM 1 and RAM 2. WhenRDDT48 through RDDT51 is a binary 1111, the nibble counter in nibble outcontrol 76 is decremented by one, and the nibble counter in nibble writecontrol 86 is decremented by one, and if either counter goes from zeroto three, the associated word counter in RAM 1 address counter 75 or RAM2 address counter 87 is decremented by one thereby allowing consecutivenibbles in RAM 1 and RAM 2 to be addressed from right to left.

The microoperations controlled by subfield F will now be discussed withreference to Table 8 and FIG. 4.

                  TABLE 8                                                         ______________________________________                                        Subfield F Microoperations                                                    RDDT Bits When ROS Address Greater Than 2K                                    RDDT Bits                                                                     52  53     54     55   Operation                                              ______________________________________                                        0   0      0      0    No Operation                                                                  (NOP)                                                  0   0      0      1    Transceivers In to CIL                                                        (CIPINN)                                               0   0      1      0    Transceivers Out to Microprocessor                                            (CIPOUT)                                               0   0      1      1    Write Zone Bits to RAM 2                                                      (CWZONE)                                               0   1      0      0    Decrement Address RAM 2                                                       (CDAD02)                                               0   1      0      1    Load Indicators                                                               (CLDFLP)                                               0   1      1      0    Increment Address RAM 2                                                       (CIAD02) = (CIPINN & CIAD02)*                          0   1      1      1    Set Carry Indicator                                                           (CSETCA)                                               1   0      0      0    Set Test Mode Flop                                                            (QLTCTL)                                               1   0      0      1    Transceivers In to CIL and                                                    Increment Address RAM 2                                                       (CINIA2)                                               1   0      1      0    Transceivers In to CIL and                                                    Decrement Address RAM 2                                                       (CINDA2) = (CIPINN & CDAD02)*                          1   0      1      1    Transceivers Out to Microprocessor                                            and Increment Address RAM 2                                                   (COAIA2) = (CIPOUT & CIAD02)*                          1   1      0      0    Transceivers Out to Microprocessor                                            and Decrement Address RAM 2                                                   (COTDA2) = (CIPOUT & CDAD02)*                          1   1      0      1    Not Used                                               1   1      1      0    Not Used                                               1   1      1      1    Not Used                                               ______________________________________                                         *Parallel microoperations created by decode PROM coding.                 

When RDDT52 through RDDT55 is a binary 0000, no operation is performedby commercial instruction logic 28. Again, as discussed above, this nooperation is performed whenever the ROS 24 address is less than 2Kbecause zero register 102 is enabled into ROS CIL register 103 asdiscussed above. When RDDT52 through RDDT55 is a binary 0001,transceivers 97 are enabled such that the data on processor bus 37 isavailable to data-in register 98 and data-in register 98 is clocked suchthat the information becomes available to commercial instruction logic28 at the output of data-in register 98. When RDDT52 through RDDT55 is abinary 0010, the transceivers 97 are enabled to transmit data fromcommercial instruction logic 28 to microprocessor 30 such that theinformation in RAM 2 data register 98 is passed to processor bus 37. Inaddition, this microoperation clocks data-in register 98 such that thedata from RAM 2 data register 88 is loaded into data-in register 98. Byloading data-in register 98 with the contents of RAM 2 data register 88,information can effectively be transferred from RAM 2 through data-inregister 98 and into RAM 1 81 by a parallel microoperation or by asubsequent microoperation. When RDDT52 through RDDT55 is a binary 0011,the zone bits at the input of result/zone multiplexer 91 are enabledonto its outputs thereby allowing the zone nibble of a binary 0011 to beloaded into one of the nibbles of a word within RAM 2 96. This is usedwhen processing string decimal data. When RDDT52 through RDDT55 is abinary 0100, the word address counter in RAM 2 address counter 87 isdecremented by one. When RDDT52 through RDDT55 is a binary 0101, thedecimal indicator register 85 is loaded with the four indicator bitsfrom decimal adder/subtractor PROM 84. This loading of indicators isnormally specified whenever an add or subtract operation is performed sothat the status of the indicator bits is latched into decimal indicators85. When RDDT52 through RDDT55 is a binary 0110, the word addresscounter in RAM 2 address counter 87 is incremented by one therebypointing to the next word within RAM 2 96. It should be noted that theincrementing and decrementing of the address counter for RAM 2 iscontrolled by subfield F and the incrementing and decrementing of theaddress counter for RAM 1 is controlled by subfield C so that theaddress counters of RAM 2 and RAM 1 can be incremented and decrementedin parallel.

When RDDT52 through RDDT55 is a binary 0111, the carry indicator indecimal indicators 85 is set to a binary ONE. This function is useful toallow a carry-in to be forced into decimal adder/subtractor PROM 84.When RDDT52 through RDDT55 is a binary 1000, a test mode flop is set toindicate that a fault condition has occurred within commercialinstruction logic 28. When RDDT52 through RDDT55 is a binary 1001,transceivers 97 are enabled to receive data from processor bus 37 anddata-in register 98 is clocked such that the data is latched in it andat the same time the word address counter in RAM 2 address counter 87 isincremented by one. By using a series of these microoperations,consecutive locations in RAM 2 96 can be loaded from the contents ofprocessor bus 37. When RDDT52 through RDDT55 is a binary 1010,transceivers 96 are enabled to receive the data from processor bus 37and it is latched into data-in register 98. In parallel, the addresscounter in RAM 2 address counter 87 is decremented by one such that thenext lower word in RAM 2 96 is addressed. As in the previousmicrooperation, it is useful for allowing consecutive words in RAM 2 tobe loaded from processor bus 37.

When RDDT52 through RDDT55 is a binary 1011, transceivers 97 are enabledto transmit the data from the output of RAM 2 data register 88 toprocessor bus 37 and at the same time this data is loaded into data-inregister 98. In addition, the address counter in RAM 2 address counter87 is incremented by one. This microoperation is useful to allowconsecutive words in RAM 2 to be transmitted to processor bus 37 andalso to allow consecutive words from RAM 2 to be loaded into data-inregister 98 from which they can be loaded by a parallel microoperationinto RAM 1 81. When RDDT52 through RDDT55 are a binary 1100,transceivers 96 are enabled to processor bus 37 to allow the outputtingof data in RAM 2 data register 88 and at the same time the data isloaded into data-in register 98. In addition, the address counter in RAM2 address counter 87 is decremented by one such that the next lower wordin RAM 2 96 is pointed to. This microoperation is also useful to allowconsecutive words in RAM 2 96 to be transmitted to processor bus 37 andloaded into data-in register 98. As in the previous microoperation, thismicrooperation is also useful to transfer consecutive words from RAM 2into RAM 1. The microoperations specified by binary 1101, 1110 and 1111are not used.

COMMERCIAL SOFTWARE INSTRUCTION DATA TYPES

The three different data types handled by the commercial instructionlogic 28 are: decimal data, alphanumeric data, and binary data. Thedecimal and binary data types are used to represent fixed point integernumeric values. The alphanumeric data type is used to representalphabetic, numeric and punctuation of text information. The commercialsoftware instructions of the CPU permit arithmetic operations on thedecimal data and editing operations on alphanumeric data. Thesecommercial software instructions are performed by commercial instructionlogic 28 in conjunction with microprocessor 30.

The single unit of information of each data type will be referred to asan "atom". Table 9 gives the size of an atom in bits as a function ofthe data type.

                  TABLE 9                                                         ______________________________________                                        Size of Data Atoms                                                                               Atom Size                                                  Data Type          In Number of Bits                                          ______________________________________                                        String (Unpacked) Decimal                                                                        8                                                          Packed Decimal     4                                                          Alphanumeric       8                                                          Binary              8*                                                        ______________________________________                                         *This means that single precision binary numbers consist of two atoms, or     16 bits, while double precision binary numbers consist of four atoms or 3     bits.                                                                    

Eight-bit atoms are also referred to as "bytes" and 4-bit atoms are alsoreferred to as "nibbles".

FIG. 7A illustrates the position of byte 0 and byte 1 within the 16-bitwords of the preferred embodiment. FIG. 7B illustrates the positions ofnibble 0 through nibble 3 in a 16-bit word. The bits are numbered 0through 15 with bit 0 being the most significant bit (MSB) and bit 15being the least significant bit (LSB) as illustrated in FIGS. 7A and 7B.

Decimal data can be represented in either string (also referred to asunpacked) or packed form. The maximum length of a decimal operand is 31atoms (i.e., if separate sign, then 30 digits plus sign). Decimalnumbers are treated as integers with the decimal point assumed to be tothe right of the least significant digit. A decimal operand can besigned or unsigned and when unsigned it is assumed to be positive.

String (unpacked) decimal digits (atoms) occupy one byte position inmemory. They can start and/or end on any byte boundaries. The four mostsignificant bits of a decimal string digit are called the zone bits. Thefour least significant bits of decimal string digit define the value ofthe digit. Only the codes 0-9 are valid for the low order four bitsotherwise an illegal character (IC) trap will result. Zone bits are notchecked by the hardware on input but will be set to 3 hexadecimal (0011binary) on output.

String decimal data can be signed or unsigned. When unsigned the operandis considered to be positive and its length refers only to digits. Whensigned, the operand can have either: leading separate sign, trailingseparate sign, or trailing overpunched sign.

The length of the operand also includes the sign character. Table 10gives the sign convention for string decimal operands having leading ortrailing signs and Table 11 gives the sign convention for string decimaloperands having trailing overpunch signs.

Packed decimal digits (atoms) occupy four bits or one-half a byteposition in memory (also referred to as nibbles). These digits can startand/or end on any half byte boundaries. The only valid codes for packeddecimal digits are 0-9 otherwise an illegal character trap will result.

Packed decimal data can be signed or unsigned. When unsigned, theoperand is considered to be positive and its length refers only todigits. When signed, the sign will be the least significant atom of theoperand. The length of the operand will include the sign atom. Table 12gives the sign conventions for packed decimal operands. Sign, whenspecified, can only be separate trailing; i.e., the rightmost atom ofthe operand field.

                  TABLE 10                                                        ______________________________________                                        Sign Conventions for String Decimal Operands                                  Having Leading And Drawing Sigl                                               SIGN                                                                          VALUE   ASCII CHARACTER HEXADECIMAL CODE                                      ______________________________________                                        +       +               2B                                                    -       -               2D                                                    ______________________________________                                         Note:                                                                         The number of digits equals L1 and the sign occupies one atom position.  

                                      TABLE 11                                    __________________________________________________________________________    Sign Convention For String Decimal Operands                                   Having Trailing Overpunch Sign                                                                  HEXADECIMAL CODE                                                              RECOGNIZED   HEXADECIMAL CODE                               SIGN DIGIT                                                                              ASCII   AND          RECOGNIZED                                     VALUE                                                                              VALUE                                                                              CHARACTER                                                                             GENERATED    ONLY                                           __________________________________________________________________________    +    0    (left brace)                                                                          7B           30                                             +    1    A       41           31                                             +    2    B       42           32                                             +    3    C       43           33                                             +    4    D       44           34                                             +    5    E       45           35                                             +    6    F       46           36                                             +    7    G       47           37                                             +    8    H       48           38                                             +    9    I       49           39                                             -    0    (right brace)                                                                         7D           None                                           -    1    J       4A                                                          -    2    K       4B                                                          -    3    L       4C                                                          -    4    M       4D                                                          -    5    N       4E                                                          -    6    O       4F                                                          -    7    P       50                                                          -    8    Q       51                                                          -    9    R       52                                                          __________________________________________________________________________     Notes:                                                                        1. For length of operand equal to L, the number of digits equals L and        sign is overpunched on rightmost digit.                                       2. The hardware uses only the low order 7bits of the byte to determine it     sign value.                                                              

                                      TABLE 12                                    __________________________________________________________________________    Packed Decimal Sign Conventions                                                            PACKED DECIMAL ASCII SIGN                                                     RECOGNIZED AND                                                   SIGN DIGIT IN                                                                              GENERATED BY                                                                              RECOGNIZED ONLY                                      HEXADECIMAL CODE                                                                           HARDWARE    BY HARDWARE                                          __________________________________________________________________________    A                        +                                                    B            +                                                                C                        +                                                    D            -                                                                E                        +                                                    F                        +                                                    __________________________________________________________________________

Alphanumeric operands consist of ASCII 7-bit characters. Their maximumsize is 255 characters except as specified otherwise. Each alphanumericcharacter atom occupies one 8-bit byte. Alphanumeric strings in memorycan start and/or end or both on either odd or even byte boundaries.

Binary operands can be either 16 bits long or 32 bits long (i.e., one ortwo words). The are 2s complement numbers and thus the most significantbit is the sign bit and the binary point is assumed to be to the rightof the least significant bit. The range (RG) of the value of a binaryoperand is:

for a 16-bit long operand: -2 to the 15th less than or equal to RG lessthan or equal to 2 to the 15th -1 and for a 32-bit long operand: -2 tothe 31st less than or equal to RG less than or equal to 2 to the 31st-1.

Note that the binary atom is eight bits and thus the length of theoperand should be either two or four atoms, otherwise unspecifiedresults will occur. Binary operands in memory can start and/or end oneither odd or even byte boundaries.

BASIC SOFTWARE INSTRUCTIONS

There are seven types of basic software instructions: generic, branch onindicators, branch on registers, shift short and shift long, short valueimmediate, input/output, single operand, and double operand. The formatfor single and double operand basic software instructions is shown inFIG. 8A.

The significance of the bits in FIG. 8A is as follows: bit 0 is always abinary ONE; bits 1, 2 and 3 are binary ZERO for single operandinstructions and define a register number (1-7) in double operandinstructions (the op code defines whether this is one of the 7 general(R) registers or one of the 7 address(B) registers; bits 4 to 8 definethe operation code; bits 9-15 are the address syllable (AS) and are usedto define either: (1) a location in memory that contains an operand, (2)a register that contains an operand, or (3) an immediate operand, wherethe operand is contained in the second word of the instruction.

Single and double operand instructions can be either one, two or threewords in length depending on the addressing mode utilized. Theaddressing mode is defined by the address syllable. Instructions thataddress a register are one word in length. Instructions that utilize animmediate operand are considered to be two words in length, includingthe operand, since the program counter is incremented by two in order toaccess the next instruction. And finally, instructions that addressoperands in memory can be either one, two or three (two, three or fourfor those requiring a mask) words in length depending on the addressingmode used.

The three address modes are: absolute, base and relative addressing.

Absolute addressing--(also called immediate address mode)--two-wordinstructions where the second word contains a 16-bit absolute address(short address format), or three word instructions where the second andthird words contain a 20-bit absolute address (long address format).

Base addressing--one-word instructions that define one of the sevenaddress registers (B1-B7) as containing the address of the operand.

Relative addressing--two-word instructions where the second wordcontains an algebraic displacement (±32K) relative to either the programcounter (P relative), an address register (base relative), or theinterrupt vector for the current central processor level.

COMMERCIAL SOFTWARE INSTRUCTIONS

The commercial software instructions of CPU 20 are classified asfollows: numeric, alphanumeric, edit and branch.

The format of commercial branch instructions is identical to that of theCPU branch instructions as shown in FIG. 8B where:

OP CODE=specifies one of the commercial software branch instructions.Bits 0, 4 and 5 are binary ZEROs, bits 6 and 7 are binary ONEs and bits1 through 3 specify which commercial indicator is to be tested and bit 8specifies if a branch is to occur when the indicator is true (i.e., abinary ONE) or false (i.e., a binary ZERO).

DISPLACEMENT=specifies the software instruction by its displacement innumber of words forward or backward from the branch instruction to whichthe branch is to transfer control if the condition is met.

The format for commercial numeric, alphanumeric and edit softwareinstructions is given in FIGS. 8C-1 through 8C-3 where:

OP CODE=specifies one of the commercial software instructions. Bits 0through 9 of the op code word are binary ZEROs, bit 10 is a binary ONEand bits 11 through 15 specify the particular commercial softwareoperation to be performed.

DDn=Data descriptor specifies the type, size and location of theoperand. DD1 refers to the first data descriptor; DD2 refers to thesecond and DD3 refers to the third.

LABEL n=12 bit displacement capable of addressing any of up to 4K remotedata descriptors. Label 1 refers to the first data descriptor; Label 2refers to the second and Label 3 refers to the third.

FIG. 8C-1 illustrates the format of a commercial software instructionusing in-line data descriptors which describe the 1 to 3 operands usedby the instruction with the number of operands being a function of thesoftware instruction type. FIG. 8C-2 illustrates the format of acommercial software instruction using remote data descriptors todescribe the 1 to 3 operands used by the software instruction and FIG.8C-3 shows the format of a commercial software instruction using acombination of in-line and remote data descriptors to describe the 1 to3 operands. The CPU distinguishes between in-line and remote datadescriptors by examining bits 12 through 15 of the first word of a datadescriptor. In remote data descriptors, bits 12 through 15 are binaryZEROs.

A commercial software instruction can have from 1 to 3 operands whichare defined by data descriptors. Data descriptors (DDs) are used todefine the operand type, size and location in memory. Data descriptorscan be in-line DDs (IDs) or remote DDs (RDs) but regardless of theirlocation in memory, they have the same format. IDs are part of acommercial software instruction (see FIG. 8C-1). RDs are defined by alabel within the commercial software instruction (see FIG. 8C-2). Thelabel is a 12-bit positive integer which is used as an offset from theremote descriptor base address which is contained in the CPU 20-bitremote descriptor base register (RDBR).

As a function of the instruction op code, a data descriptor can beeither a: decimal DD, alphanumeric DD, or binary DD.

Decimal DDS are implied by all numeric software instructions and thenumeric edit instruction. Decimal DDs can refer to either string decimalor packed decimal data. The format of the DD is as shown in FIG. 9where:

WORD 1:

C1, C2, C3,=control bits and specify atom offset and sign information.

L=specifies the length of the operand in number of atoms.

T=specifies the data type:

If T=binary ZERO, data is string (unpacked) decimal and

IF T=binary ONE, data is packed decimal.

CAS=specifies the commercial address syllable (see CAS descriptionbelow).

WORD 2:

The contents of word 2 is either a displacement or an immediate operand(IMO) (i.e., the operand itself instead of a pointer to the operand) asdefined by the CAS.

In a string decimal DD the fields, shown in FIG. 9, have the followingmeaning:

1. C1=byte (atom) offset.

a. When no indexing is specified, C1 specifies the offset within theaddressed word:

C1=binary ZERO, operand starts in the left most byte of the addressedword.

C1=binary ONE, operand starts in the right most byte of the addressedword.

b. When indexing is specified, C1 contains an atom offset value that isadded to the index value and the resulting sum is used to compute theeffective address of the operand.

2. C2, C3=sign control as shown in the following tabulation:

    ______________________________________                                        C2     C3        SIGN CONVENTION                                              ______________________________________                                        0      0         Unsigned (assumed to be positive)                            0      1         Trailing Overpunch                                           1      0         Leading Separate Sign                                        1      1         Trailing Separate Sign                                       ______________________________________                                    

3. L=Specifies the length of the operand either directly or indirectly:

for L not equal to 0, then 1 less than or equal to L less than or equalto 31 for L=0, the escape to CPU registers 11-15, where:

Register 4 for DD1

Register 5 for DD2

Register 6 for DD3.

Register 8-10 should be zero else unspecified results will occur.

Note that for unsigned or sign overpunched operands, the length refersto number of digits. For a leading or trailing separate signed operand,its length refers to L-1 digits and a sign.

Note that an illegal specification trap will result if an operand haseither: a length of 0, or a length of 1 and specifies separate sign(i.e., the operand consists only of a sign).

4. T=binary ZERO.

5. CAS=specifies the commercial address syllable.

In a Packed Decimal DD the fields shown in FIG. 9 have the followingmeaning:

1. C1, C2=Nibble (atom) offset.

a. When no indexing is specified then C1 and C2 specify the offsetwithin the addressed word:

    ______________________________________                                                       POSITION OF FIRST DIGIT WITHIN                                 C1    C2       THE ADDRESSED WORD                                             ______________________________________                                        0     0        Nibble 0 (bits 0-3)                                            0     1        Nibble 1 (bits 4-7)                                            1     0        Nibble 2 (bits 8-11)                                           1     1        Nibble 3 (bits 12-15)                                          ______________________________________                                    

b. When indexing is specified C1 and C2 contain an atom offset valuewhich is added to the index value and the resulting sum is used tocompute the effective address of the operand.

2. C3=Sign control:

If C3=binary ZERO, the operand is unsigned and

If C3=binary ONE, the operand is trailing sign.

When unsigned, the operand is considered to be positive.

When signed, only trailing sign is allowed.

3. L=specifies the length of the operand directly or indirectly. Thoserules under string decimal DD for L also apply here.

4. T=binary ONE.

Alphanumeric DDs are implied by all alphanumeric software instructionsand the alphanumeric edit software instruction. The format of thealphanumeric DD is somewhat similar to that shown in FIG. 9 but need notbe further described.

Commercial software instructions generate address references through afiled called the commercial address syllable (CAS). The resolution ofthe CAS field for non branch instructions usually results in theformation of an effective address (EA) which points to an operand butcan also directly describe an operand (i.e., an immediate operand).

The CAS is a seven bit field of a data descriptor in which bits 9through 11 are used as an address modifier, bit 12 is generally used toindicate whether direct or indirect addressing is to be used, and bits13 through 15 are used to indicate a base register number. The CAS ofFIG. 9 should not be confused with the AS used by the basic software CPUinstructions of FIG. 8A although they are similar.

The general rules describing the commercial address syllable entries areas follows:

1. An in-line descriptor (ID) is a DD that is part of the instruction.

2. An ID can either point directly to the operand or to a pointer to theoperand (i.e., whenever indirection is specified).

3. A remote descriptor (RD) is a DD defined by a label (which is part ofthe instruction) and the remote descriptor base register.

4. An RD can either point directly to the operand or to a pointer to theoperand (i.e., when indirection is specified).

5. During extraction of commercial software instructions, the CPU mustdetermine whether IDs or RDs are used.

6. The contents of the program counter and base registers are consideredword addressed.

7. An index register content is considered an atom displacement and hasa range of minus 2 to the 15th less than or equal to the index registercontent which is less than or equal to 2 to the 15th minus 1.

8. The displacement is a word displacement and has a range of minus 2 tothe 15th less than or equal to the displacement which is less than orequal to 2 to the 15th minus 1.

9. When program counter plus displacement addressing is specified thenthe program counter contents means the address of the word containingthe displacement.

10. An immediate operand (IMO) always occupies the second word of a DD.Its offset and length are specified in the DD. For offset plus lengthspecifying more atoms than can be contained in a word, then unspecifiedresults will occur. Note that a decimal IMO can specify any signconvention supported for normal operands.

The definition of the commercial software instructions supports postindexing of data at the atom level. The index value is in atoms and itis automatically adjusted to correspond to the data type specified inthe DD.

During effective operand address generation, the atom index defined inone of seven index registers (R1, R2, . . . R7), may be added to theatom offset in the DD before this is algebraically added as the laststep (after any indirection) to the base address.

The following general rules will apply to all numeric commercialsoftware instructions:

1. Effective address (EA) always points to the leftmost atom of theoperand.

2. Greater than (G) or less than (L) indicators indicate value of resultrelative to zero except for convert decimal to binary (CDB), and convertbinary to decimal (CBD) instructions.

3. If result is shorter than receiving field, the receiving field willbe zero filled to the left.

4. Plus and minus zero are allowed on input but will be assumed to beplus zero during instruction execution. All zeros operands generated bythe hardware will be plus zeros.

5. A string decimal zero=30 (hexadecimal); a packed decimal zero=0(hexadecimal).

6. All signed results will have the hardware generated signs, regardlessof the sign convention used by the operands at execution time.

7. Zone bits (leftmost four bits of a string decimal number) are alwaysignored on input. Nevertheless, all results will have the zone bits setto 3 hexadecimal.

8. Operands must not overlap each other, otherwise unspecified resultswill occur.

9. Indentical overlapping operands are allowed.

10. If a trap condition is disabled the trap will not occur and thereceiving field will be altered.

11. Mixing of operands (i.e., string or packed decimal) is allowed.

12. Operands having different signs conventions are allowed.

13. Any operand having a zero length or a length equal to just the signcharacter, and specifying separate sign, will result in an illegalspecification (IS) trap condition.

14. Overflow (OV) indicator is set if the receiving field cannot acceptall significant digits of the result.

15. If either operand has an illegal digit or an illegal sign, then anillegal character (IC) trap will result.

16. If a negative operand or a negative result is to be stored in anunsigned receiving field, then the sign fault indicator is set and notrap occurs.

17. If DD2 specifies IMO in any instruction except a decimal compare(DCM) instruction, an IS trap will occur.

18. If DD3 specifies IMO, an IS trap will occur.

19. Whenever a trap occurs, the state of the indicators used by thetrapped instruction will be unspecified.

20. The original operands preservation cannot be guaranteed whenevereither an unavailable resource trap or a bus or memory error trapoccurs. If any other trap occurs, then the operands will remainunchanged.

21. If the receiving field cannot accept all significant digits of theresult and commercial instruction mode register (CM) specifies that notrap be generated then the receiving field will contain only low orderdigits (i.e., the high order digits will be lost).

NUMERIC COMMERCIAL SOFTWARE INSTRUCTIONS

The numeric commercial software instructions of interest are describedbelow.

Decimal add (DAD) adds the contents DD1 (i.e., the operand pointed to bydata descriptor 1) to the contents of DD2 and the result replaces thecontents of DD2 (i.e., [DD2]+[DD1] stored in [DD2], augend+addend=sum).The overflow (OV), sign fault (SF), greater than (G) and less than (L)commercial indicators will be set as a function of the result.

Decimal subtract (DSB) subtracts the contents of DD1 from contents ofDD2 and the result replaces the contents of DD2 (i.e., [DD2]-[DD1]stored in [DD2], minuend-subtrahend=difference). The OV, SF, G and Lcommercial indicators are set as a function of the result.

Decimal multiply (DML) multiplies the contents of DD2 by the contents ofDD1 and the result replaces the contents of DD2 (i.e., [DD2] * [DD1]store in [DD2], multiplicand×multiplier=product).

Decimal divide (DDV) divides the contents of DD2 by the contents of DD1and the resulting quotient is stored in the contents of DD3 and theremainder replaces the contents of DD2 (i.e., [DD2]/[DD1] stored in[DD3] and remainder stored in [DD2], dividend/divisor=quotient andremainder). The OV, SF, G, and L commercial indicators are set as afunction of the result. Overflow occurs if quotient is larger than DD3receiving field or if a divide by zero is attempted. The sign of thequotient is determined by rules of algebra and sign of remainder isequal to sign of the dividend unless the remainder is zero.

Decimal compare (DCM) makes an algebraic comparison of the contents ofDD1 to contents of DD2 and sets commercial indicators to indicate ifcontents of DD1 are equal, greater, or less than contents of DD2 (i.e.,[DD1]::[DD2] results to commercial indicators). The G and L commercialindicators are set as function of comparison.

Convert binary to decimal (CBD) moves and converts the contents ofbinary DD1 and places the decimal string or packed result in thecontents of DD2 (i.e., [DD1] converted and stored in [DD2]). The OV andSF commercial indicators are set as a function of the result.

Convert decimal to binary (CBD) moves and converts the contents of adecimal (string or packed) DD1 and places the binary result in thecontents of DD2 (i.e., [DD1] converted and stored in [DD2]). The OVcommercial indicator is set as a function of the result.

COMMERCIAL INSTRUCTION LOGIC DETAILS

Before decribing the manner in which the commercial instruction logic 28is used to perform the various decimal operations, the structure ofcommercial instruction logic 28 will be described in greater detail withrespect to FIGS. 10A through 10D. FIG. 10A primarily shows the structureof RAM 1 81 and its associated addressing logic, FIG. 10B primarilyshows the structure of RAM 2 and its associated addressing logic, FIG.10C primarily shows the structure of decimal adder/subtractor PROM 84along with its inputs and outputs and decimal indicators 85, and FIG.10D primarily shows the structure of CIL control area 100.

In FIGS. 10A through 10D, the same reference numerals used previouslyare shown in addition to the detailed circuitry required for theoperation of the hardware mechanism. In FIGS. 10A through 10D the littlecircles at some of the inputs and outputs of the various elements areused to represent inverting inputs or outputs respectively.

RAM 1 LOGIC DETAILS

The operation of RAM 1 81 will now be described with reference to FIG.10A. In the preferred embodiment, transceivers 97 is comprised of twoTexas Instruments (TI) type SN74LS245 octal bus transceivers withthree-state outputs as described in their publication, The TTL Data Bookfor Design Engineers, Second Edition, copyright 1976 by TexasInstruments, Incorporated of Dallas, Tex., which reference isincorporated herein by reference. Transceivers 97 operates such thatwhen the direction input signal CIPINN- at its DIR input is a binaryZERO, the data at the D inputs is transmitted to the Q outputs (i.e.,the signals on line CPBX00+ through CPBX15+ is transmitted to the linesCIPB00+ through CIPB15+) and when the DIR signal is in the binary ONEstate, the data at the Q inputs is transmitted to the D outputs suchthat the signals on line CIPB00+ through CIPB15+ is transmitted to lineCPBX00+ through CPBX15+. Signals CPBX00+ through CPBX15+ come fromprocessor bus 37 in microprocessor 30. The direction signal CIPINN-comes from a decode of microinstruction bits RDDT52 through RDDT55 asperformed by decoder 105 (see FIG. 10D). The D and Q inputs and outputsof transceivers 97 are isolated whenever the function (F) input is abinary ONE as determined by signal CINOUT- at the output of NAND gate110. Signal CINOUT- is produced by NANDing together signal PHASEA- andsignal CINOUT+. Signal PHASEA- is produced by inverting clocking signalPHASEA+ from microprocessor 30 via inverter 112. Signal CINOUT+ isproduced by combining signal CIPOUT- and CIPINN- from decoder 105 viaNAND gate 111.

Signals CIPB00+ through CIPB15+ are wire-ored together at point 113 withsignals CD2L00+ through CD2L15+ to produce signals CIPI00+ throughCIPI15+. Thus, transceivers 97 can input to data-in register 98 the datafrom processor bus 37 or can transmit to processor bus 37 the data fromRAM 2 data register 88. When data-in register 98 is clocked by clockingsignals CINNLD+ at its clock (C) input transitioning from the binaryZERO to the binary ONE state, the data at its D0 through D15 inputs islatched into the register and will appear at the Q0 through Q15 outputsas signals CBUS00+ through CBUS15+. Thus, data-in register 98 can storeeither the output of transceivers 97 when it is enabled or the output ofRAM 2 data register 89 when it is enabled (see FIG. 10B). Load controlsignal CINNLD+ is produced by AND gate 108 combining signal CINOUT+ fromNAND gate 111 and clocking signal PHASEB+ from microprocessor 30.

In the preferred embodiment, RAM 1 81 is comprised of four type AM74S189manufactured by Advanced Micro Devices, Inc. of Sunnyvale, Calif., anddescribed in their publication entitled, Advanced Micro DevicesBipolar/MOS Memories Data Book, copyrighted 1982, which is incorporatedherein by reference, random access memories having 16 words of 4 bitseach with three-state outputs. The four address (ADR) inputs areconnected to RAM 1 address counter 75 to receive signals CAD100+ throughCAD103+ such that the same four-bit word will be accessed in each of theRAMS. The memory data inputs D0 through D3 are connected to receive fourbits from data-in register 98 such that RAM 1-0 81-0 receives bitsCBUS00+ through CBUS03+, RAM 1-1 81-1 receives bits CBUS04+ throughCBUS07+, RAM 1-2 81-2 receives bits CBUS08+ through CBUS11+ and RAM 1-381-3 receives bits CBUS12+ through CBUS15+. Each RAM chip is writeenabled by signal CWROP1- at its write enable (WE) input. The Q outputsof each RAM 1 chip are individually enabled under the control of the Finput such that at any given time only one of the four RAMS will haveits four bits output enabled. The output of RAM 1-0 81-0 is controlledby signal CSEL10- from AND gate 76-3, RAM 1-1 81-1 has its outputscontrolled by signal CSEL11- from AND gate 76-4, RAM 1-2 81-2 has itsoutputs controlled by signal CSEL12- from AND gate 76-5 and RAM 1-3 81-3has its outputs controlled by signal CSEL13- from AND gate 76-6. ANDgates 76-3 through 76-6 all have signal CWROP1- at one input. The secondinput of each of these AND gates comes from decoder 76-2 as signalCRD100- through signal CRD103-. Of these four signals, only one will bein the binary ONE state at any given time such that only one of signalsCSEL10- through CSEL13- will be in the binary ZERO state at any giveninstance thereby allowing the Q outputs of RAM 1-0 through RAM 1-3 to bewire-ored together at point 114.

The four bit word which is enabled at the output of one of RAM 1-0through RAM 1-3 is determined by nibble out control 76 logic which iscomprised of RAM 1 nibble counter 76-1, decoder 76-2 and AND gate 76-3through 76-6. The 16-bit word which is written in four bits into each ofRAM 1-0 through RAM 1-3 is controlled by RAM 1 address counter 75. Bycontrolling RAM 1 in this manner, a 16-bit word can be written into RAM1 in the word addressed by RAM 1 address counter 75 and a 4-bit nibblecan be output as selected by RAM 1 address counter 75 and nibble-outcontrol 76. RAM 1 81 is write enabled by signal CWROP1- from AND gate115. AND gate 115 combines the clocking signals PHASEA+ and PHASEB+ frommicroporcessor 30 along with signal CWRT01+ which is output by AND gate116. AND gate 116 combines signal CIPCOD+ and signal CROS41+. SignalCROS41+ comes from microinstruction bit RDDT41 and signal CIPCOD+ isderived by combining microinstruction bits CROS35+ through CROS37+ inNAND gate 117 the output of which is signal CIPCOD- which is inverted byinverter 118 to produce signal CIPCOD+.

In the preferred embodiment, RAM 1 address counter 75 and RAM 1 nibblecounter 86-1 are TI type SN74LS169 synchronous 4-bit up/down counters asdescribed in The TTL Data Book for Design Engineers. Both counters areclocked each microinstruction when signal PHASEB- transitions from thebinary ZERO to the binary ONE state. Signal PHASEB- is derived byinverting clocking signal PHASEB+ from microprocessor 30 by use ofinverter 119. When both count enable inputs P and T are in the binaryZERO state, the clocking of the counter will result in the counter beingincremented by one or decremented by one depending upon the state of theup/down (U/D) input. RAM 1 address counter 75 is count enabled by signalCNTOP1- being in the binary ZERO state and RAM 1 nibble counter 76-1 iscount enabled by signal CNTCT1- being in the binary ZERO state. RAM 1address counter 75 counts up when signal CDWN01- is a binary ONE andcounts down when that signal is a binary ZERO. Similarly, this RAM 1nibble counter 76-1 counts up when signal CTDCT1- is a binary ONE andcounts down when that signal is a binary ZERO. Signal CDWN01- is outputby AND gate 120 which combines signals CDAD01- from decoder 106 andsignal CTDCT1- from decode PROM 104 both of which are derived fromdecoding various bits of the microinstruction word (see FIG. 10D). RAM 1address counter 75 is loaded with bits CBUS13+ through CBUS10+ whensingla CLDAD1- at the load (L) input is in the binary ZERO state. SignalCLDAD1- is derived by decoding microinstruction bits via decoder PROM104 (See FIG. 10D). Similarly, RAM 1 nibble counter 76-1 is loaded withbits CBUS15+ and CBUS14+ when under the control of signal CLDCT1- at itsload (L) input. Signal CLDCT1- also comes from decode PROM 104 (see FIG.10D). The enabling of RAM 1 nibble counter 76-1 is controlled by signalCNTCT1- which is the output of AND gate 121. AND gate 121 combinessignal CTUCT1- and signal CTDCT1- both of which are derived from themicroinstruction bits being decoded by decode PROM 104.

The 2-bit output of RAM 1 nibble counter 76-1 which allows it to countfrom 0 through 3 are signal CNT101+ and CNT100+. These two signals aredecoded by decoder 76-2 such that the one chip of RAM 1-0 through RAM1-3 will have its outputs enabled which are ORed together by wire-or 114to produce signal CRAM10- through CRAM13-. These signals are input toRAM 1 zero multiplexer 82 to provide one of two inputs to be selectedbetween. The other inputs to RAM 1 zero multiplexer 82 is a binary ONEwhich when inverted produces a binary ZERO at its output if that inputis selected. RAM 1 zero multiplexer 82 is always enabled by the binaryZERO appearing at its enable (F) input. The selection between the twoinputs is done under the control of select (SEL) input CROS47- which isderived by inverting microinstruction bit CROS47+ by inverter 122. Theoutput of RAM 1 zero multiplexer 82, signals COP100+ through COP103+, isinput into double multiplexer 83 (see FIG. 10C).

Gates 123 through 128 control the enabling of RAM 1 address counter 75which can be enabled to either count up or count down each time amicroinstruction is executed. Gate 120 controls the direction in whichthe address counter 75 will be counted, whether it is up or down. Theinputs to AND gate 120 are signals CTDCT1- and CDAD01-. Signal CTDCT1-will be a binary ONE if a CTDCT1 microoperation is included within themicroinstruction thereby indicating that RAM 1 nibble counter 76-1 is tobe decremented (i.e., counted down) and signal CDAD01- will be a binaryZERO if a CDAD01 microoperation is included in a microinstructionthereby indicating that RAM 1 address counter 75 is to be decremented.Thus, the output of AND gate 120, signal CDWN01-, will be a binary ZEROif either RAM 1 nibble counter 76-1 or RAM 1 address counter 75 is to becounted down, thus setting RAM 1 address counter 75 into the decrementmode by providing a binary ZERO signal at its U/D input.

The enabling of RAM 1 address counter 75 is done by control signalCNTOP1- being in the binary ZERO state at the output of AND gate 128.There are three conditions which will cause the output of AND gate 128to be a binary ZERO and thus enable RAM 1 address counter 75 to beeither decremented or incremented depending upon the state of the U/Dinput. If either a increment address 1 microoperation (CIAD01) or adecrement address 1 microoperation (CDAD01) is encoded in themicroinstruction then signal CIAD01- will be a binary ZERO or signalCDAD01- will be a binary ZERO and the output of AND gate 127, signalCNTWD1-, will be a binary ZERO and thereby the cause the output of ANDgate 128 to be a binary ZERO and enable the incrementing or decrementingof RAM 1 address counter 75. Thus, if the microinstruction containseither an increment or decrement address 1 microoperation, RAM 1 addresscounter 75 will be enabled. The other two conditions which enable RAM 1address counter 75 are if RAM 1 nibble counter 76-1 has been decrementedto a count of 0, or has been incremented to a count of 3 thus requiringthat the word address be decremented or incremented, respectively, by 1.The output of OR gate 123, signal CNT1E0-, will be a binary ZERO if theoutput of RAM 1 nibble counter 76-1 is a binary ZERO. The binary ZERO ofsignal CNT1E0- at one input of OR gate 124 will cause signal CNTDN1- tobe a binary ZERO if its other input, signal CTDCT1-, is a binary ZERO.Signal CTDCT1- from decode PROM 104 (see FIG. 10D) will be a binary ZEROif a count down counter 1 microoperation (CTDCT1) is present in themicroinstruction, or if a count down both counter 1 and counter 2microoperation (CTDALL) is present in the microinstruction. Thus, ifwhen counting down the output of RAM 1 nibble counter 76-1 is a binary00, signal CNTDN1- will be a binary ZERO and cause the output of ANDgate 128 to become a binary ZERO and enable the counting of RAM 1address counter 75. Similarly, if counting up and the nibble counterreaches a count of 3, the output of NAND gate 125, signal CNT1E3-, willbecome a binary ZERO and if counting up, as indicated by signal CTUCT1-being a binary ZERO, the output of OR gate 126, signal CNTUP1-, willbecome a binary ZERO and cause the output of AND gate 128 to become abinary ZERO and enable RAM 1 address counter 75. Signal CTUCT1-, whichis input to OR gate 126, comes from decode PROM 104 (see FIG. 10D) andwill be a binary ZERO if either a count up counter 1 microoperation(CTUCT1) or a count up both counter 1 and counter 2 microoperation(CTUALL) is present in the microinstruction.

From this discussion it can be appreciated that RAM 1 address counter 75is enabled if the address counter 75 is to be incremented or decrementedby 1 as determined by a microinstruction and thus incremented to addressthe next word in RAM 1 81 or if counter 1 or counters 1 and 2 are beingincremented or decremented and the output of the nibble counter 76-1 isequal to 3 when counting up or equal to 0 when counting down so that thenext nibble can be addressed from the next word.

RAM 1 nibble counter 76-1 is enabled to count by signal CNTCT1- at theoutput of AND gate 121 which will be a binary ZERO if either of itsinputs, signal CTUCT1- or CTDCT1-, are a binary ZERO and the state ofthese inputs is controlled by the count up and count downmicrooperations as mentioned before. The counting direction of RAM 1nibble counter 76-1 is controlled by signal CTDCT1- and will be a binaryZERO indicating that the counter is to be decremented if a count downcounter 1 microoperation (CTDCT1) or a count down counter 1 and counter2 operation (CTDALL) is present in the microinstruction.

From the above discussion it can be appreciated that the logic in FIG.10A permits commercial instruction logic 28 to either receive data fromor send data to processor bus 37 via transceivers 97, to latch data intointo data-in register 98 from either processor bus 37 or RAM 2 dataregister 88 and that RAM 1 81 can be addressed under the control of RAM1 address counter 75 such that a 16-bit word can be written into RAM 181 or a 4-bit nibble can be read from RAM 1 81 under the control of RAM1 address counter 75 and RAM 1 nibble counter 76-1. Further, RAM 1address counter 75 can be loaded from data-in register 98 as can RAM 1nibble counter 76-1 and that RAM 1 address counter 75 can be incrementedor decremented and that RAM 1 nibble counter 76-1 can be incremented ordecremented and that incrementing through a count of 3 will also resultin the incrementing of RAM 1 address counter 75 and a decrementingthrough a count of 0 will result in the decrementing of RAM 1 addresscounter 75. Further, RAM 1 zero multiplexer 82 allows either the 4-bitnibble from RAM 1 81 to be passed on to an input of double multiplexer83 or a zero nibble to be passed on to double multiplexer 83.

RAM 2 LOGIC DETAILS

FIG. 10B illustrates the logic associated with RAM 2 96. RAM 2 96differs from RAM 1 81 in that in addition to being able to write a wholeword of 16 bits into RAM 2 with the address specified by RAM 2 addresscounter 87, a single 4-bit nibble can be written into RAM 2 96 with theaddress of the word being specified by RAM 2 address counter 87 and theone of four nibbles being selected by nibble write control 86. Inaddition, unlike RAM 1 81 which outputs only a single nibble at a time,RAM 2 96 always outputs a full 16-bit word into RAM 2 data register 88and one of four nibbles from that word is selected by RAM 2 nibblemultiplexer 89.

In the preferred embodiment, RAM 2 96 is comprised of four random accessmemory chips of the type 93422 manufacuted by Fairchild Camera andInstruments Corporation of Mountain View, Calif., and described in theirpublication, Bipolar Memory Data Book, copyrighted 1979, which isincorporated herein by reference. The word within RAM 2 96 which is tobe either written or read is controlled by connecting the output of RAM2 address counter 87, signal CAD200+ through CAD207+, to the addressinputs of RAM 2-0 chip 96-0, RAM 2-1 chip 96-1, RAM 2-2 chip 96-2 andRAM chip 96-3. The outputs of RAM 2 chips 96-0 through 96-3 are enabledby connecting the function (F) input of each chip to a binary ZERO. Thewrite enable (WE) input of RAM 2 chips 96-0 through 96-3 are controlledby signals CWR200- through CWR203- thus allowing each chip to beindividually write enabled or all chips to write enabled as will bedescribed below. The chip select (CS) inputs of RAM 2 chips 96-0 through96-3 are enabled by connecting the inverted input to a binary ZERO andthe noninverted input to a binary ONE thus enabling the chips at alltimes.

The four data input bits of the RAM 2 chips are connected to the outputof their corresponding nibble multiplexer such that, for example, thefour data inputs to RAM 2-0 chip 96-0 are connected to receive signalC2IN00+ through C2IN03+, which are the outputs of nibble 0 multiplexer92. The outputs of nibble multiplexer 92 through 95 are always enabledby connecting the function (F) input of each multiplexer to a binaryZERO. The zeroth inputs of nibble multiplexers 92 through 95 areconnected to receive a different four bits from data-in register 98 suchthat nibble 0 multiplexer 92 is connected to receive bits CBUS00+through CBUS03+, nibble 1 multiplexer 93 is connected to receive signalsCBUS04+ through CBUS07+ and similarly nibble 2 multiplexer 94 receivessignals CBUS08+ through CBUS11+ and nibble 3 multiplexer 95 receivessignals CBUS12+ through CBUS15+.

The selection between the zeroth input and first input of nibblemultiplexers 92 through 95 is controlled by signal CWROP2- at theirselect (SEL) input. Signal CWROP2- at the output of NAND gate 130 willbe a binary ZERO if a write all of RAM 2 microoperation (CWROP2) ispresent in the microinstruction thereby causing signal CROS40+ to be abinary ONE and signal CIPCOD+ to also be a binary ONE. Thus, when awrite all of RAM 2 microoperation is present, nibble multiplexers 92through 95 will select as input to RAM 2 chips 96-0 through 96-3 theoutput of data-in register 98. When signal CWROP2- is a binary ZERO, theoutput of NAND gates 131 through 134, signals CWR200+ through CXWR203+,will all be binary ONEs and partially enable NAND gates 135 through 138,respectively. The output of NAND gates 135 through 138, signals CWR200-through CWR203-, will become binary ZEROs and write enable RAM 2 chips96-0 through 96-3 when clocking signals PHASEA+ and PHASEB+ frommicroprocessor 30 become binary ONEs. Thus, it can be appreciated when awrite all of RAM 2 microoperation (CWROP2) is executed, RAM 2 chips 96-0through 96-3 will all be enabled and a different 4-bit nibble fromdata-in register 98 will be written into each chip via nibblemultiplexers 92 through 95.

Input 1 of nibble multiplexers 92 through 95 are each connected toreceive the four signals CRES00+ through CRES03+ which is output fromresult/zone multiplexer 91 (see FIG. 10C). The first input of nibblemultiplexers 92 through 95 is selected if signal CWROP2- is a binary ONEindicating that a write all of RAM 2 microoperation is not present inthe microinstruction. If a write nibble to RAM 2 microoperation (CWRES2)is present in a microinstruction, signal CROS39+ at one input of NANDgate 139 will be a binary ONE as will signal CIPCOD+ at the other inputof NAND gate 139 thus causing its output, signal CWRES2-, to become abinary ZERO and enable decoder 86-2. Decoder 86-2 decodes the nibblecount from RAM 2 nibble counter 86-1 thus enabling one of its fouroutputs, signal CWR200- through CWR203- to become a binary ZERO with theother three outputs remaining in the binary ONE state. Whichever signalof CWR200- through CWR203- becomes a binary ZERO, it will disable one ofNAND gates 131 through 134 and thereby cause the output of one of theNAND gates to become a binary ONE which will enable one of NAND gate 135through 138 and cause one of their outputs, signal CWR200- throughCWR203-, to become a binary ZERO when clocking signals PHASEA+ andPHASEB+ from microprocessor 30 become binary ONEs. With one of NANDgates 135 through 138 fully enabled and the other three NAND gatesdisabled. One of signals CWR200- through CWR203- will become a binaryZERO and write enable its corresponding RAM 2 chip 96-0 through 96-3 andthereby allow one nibble to be written into the word addressed by RAM 2address counter 87 and the nibble addressed by RAM 2 nibble counter86-1. As indicated before, the source of the single nibble to be writteninto RAM 2 96 comes from the output of result/zone multiplexer 91 assignals CRES00+ through CRES03+.

The word to be written into or read from RAM 2 96 is determined by theoutput of RAM 2 address counter 87 bit CDA200+ through CDA207+. RAM 2address counter 87 and RAM 2 nibble counter 86-1 are TI type SN74LS1694-bit up/down synchronous counters. RAM 2 nibble counter 86-1 isconnected to RAM 2 address counter 87 such that when the counter iscounting upa count of 3 in the RAM 2 nibble counter 86-1 will result inan incrementing of RAM 2 address counter 87 and when counting down acount of 0 in RAM 2 nibble counter 86-1 will result in the decrementingof RAM 2 address counter 87 in a manner similar to that described forRAM 1 nibble counter 76-1 and RAM 1 address counter 75.

The clocking of RAM 2 address counter 87 and RAM 2 nibble counter 86-1is in response to signal PHASEB- at their clock (C) input transitioningfrom the binary ZERO to the binary ONE state. RAM 2 address counter 87can be loaded with signals CBUS13+ through CBUS05+ from data-in register98 when signal CLDAD2- at the load (L) input is in a binary ZERO state.Signal CLDAD2- which comes from decode PROM 104 will be in the binaryzero state if the microinstruction contains a load address 2microoperation (CLDAD2). Similarly, RAM 2 nibble counter 86-1 will beloaded with the signals CBS15+ and CBUS14+ if signal CLDCT2- is a binaryZERO at its load (L) input. Signal CLDCT2- from decode PROM 104 will bein the binary ZERO state if a load counter 2 microoperation (CLDCT2) isspecified in the microinstruction.

The enabling of the counting of RAM 2 address counter is controlled bysignal CNTOP2- at the P and T count enable inputs. Signal CNTOP2- willbe a binary ZERO state if any one of the three inputs to AND gate 148 isa binary ZERO. Signal CNTWD2- which is one input to AND gate 148 will bein the binary ZERO state if either input to AND gate 147 is in thebinary ZERO state. One input to AND gate 147 is signal CDAD02- whichwill be a binary ZERO if a decrement address 2 microoperation (CDAD02)is specified in the microinstruction. Similarly, the output of AND gate147 will be in the binary ZERO state if an increment address tomicrooperation (CIAD02) is specified in the microinstruction which willcause signal CIAD02- to be in the binary ZERO state at the output ofdecode PROM 104 (see FIG. 10D) which is input to AND gate 147.

The other two conditions that can cause the enabling of RAM 2 addresscounter to count up or down is if the output of OR gate 146, signalCNTUP2-, or the output of OR gate 144, signal CNTDN2-, is a binary ZERO.The output of OR gate 146 will be a binary ZERO if the output of RAM 2nibble counter 86-1 is a 3 and a count up counter 2 microoperation(CTUCT2) is specified which will cause signal CTUCT2- to be in thebinary ZERO state. Similarly, the output of OR gate 144 will be in thebinary ZERO state if the output of RAM 2 nibble counter 86-1 is a 0which will cause the output of OR gate 143, signal CNT2E0-, to be abinary ZERO and if signal CTDCT2- is a binary ZERO indicating that acount down counter 2 microoperation (CTDCT2) has been specified in themicroinstruction.

RAM 2 nibble counter 86-1 is enabled to count if either a count upcounter 2 microoperation (CTUCT2) or a count down counter 2microoperation (CTDCT2) has been specified such that signal CTUCT2- willbe a binary ZERO or signal CTDCT2- at the input of AND gate 142 will bea ZERO which in turn will cause the output thereof, signal CNTCT2-, tobe a binary ZERO. The direction of counting of RAM 2 address counter 87is controlled by the output of AND gate 140, signal CDWN02-, which willbe a binary ZERO either a count down address 2 or a count down counter 2microoperation has been specified (CDAD02 or CTDCT2) such that eithersignal CDAD02- or signal CTDCT2- is a binary ZERO. The up/down directionof counting of RAM 2 nibble counter 86-1 is controlled by signal CTDCT2-which will be in the binary ZERO state and cause the counter 2 countdown if a count down counter 2 microoperation (CTDCT2) has beenspecified in the microinstruction.

If a write of a single nibble is microoperation (CWRES2) is specified inthe microinstruction, signal CROS39+ will be a binary ONE and signalCIPCOD+ will be a binary ONE at the inputs of NAND gate 139 causing theoutput thereof, signal CWRES2-, to be a binary ZERO and enable decoder86-2 such that one output will be in the binary ZERO state and the otherthree outputs will be in the binary ONE state and thereby enable threeof NAND gate 131 through 134 and disable the remaining NAND gate suchthat only one of the output signals of NAND gates 135 through 138 willbe a binary ZERO to write enable one of RAM 2 chips 96-0 through 96-3 asdescribed above.

The data read from RAM 2 96 will be enabled on to the Q outputs of RAM 2data register 88 whenever clocking signal PHASEB- is a binary ONE at theC input and the output control signal CIPINN+ is a binary ZERO at the Finput. RAM 2 data register 88 is a transparent latch which means thatthe outputs will follow the inputs as long as the C input remains in thebinary ONE state and the outputs will be latched at the level of theinputs whenever the C input becomes a binary ZERO. When the F inputsignal becomes a binary ONE, the state of the C input is ignored and theQ outputs assume a high impedance state. This permits the F input tocontrol the latching of the data into RAM 2 data register 88 such thatwhenever a microoperation is specified which indicates that the data isto be taken from processor bus 37 into transceivers 97, for example,when a CIPINN or a CINDA2 microoperation is specified, signal CIPINN-from decode PROM 105 will be in the binary ZERO state and cause theoutput of inverter 149, signal CIPINN+, to be in the binary ONE state atthe F input of RAM 2 data register 88.

Each 4-bit nibble grouping of output signals from RAM 2 data register 88is one of the four data inputs to RAM 2 nibble multiplexer 89 such thatthe four bits output from RAM 2-0 chip 96-0 are input into the zeroinput and the four bits output by RAM 2-1 chip 96-1 are input into thefirst input, etc. RAM 2 multiplexer 89 receives selection signalsCNT200+ and CNT201+ at the select (SEL) inputs from the output of RAM 2nibble counter 86-1 such that one of four 4-bit input groups will beenabled on to the Q output to produce signal COP200+ through COP203+.The nibble output by RAM 2 nibble multiplexer 89 goes to RAM 2 zeromultiplexer 90 and double multiplexer 83 (see FIG. 10C). The output ofRAM 2 nibble multiplexer 89 is always enabled because of the binary ZEROat the function (F) input. The 16 bits output by RAM 2 data register 88,signals CD2L00+ through CD2L15+, besides going to RAM 2 nibblemultiplexer 89 also go to transceivers 97 and data-in register 98 asdescribed above.

DECIMAL ARITHMETIC LOGIC UNIT DETAILS

FIG. 10C illustrates the decimal add/subtract PROM 84, decimalindicators 85, sign generator PROM 78 and associated multiplexers. Inthe preferred embodiment, decimal add/subtract PROM 84 is a type 82S191PROM manufactured by Signetics Corporation of Sunnyvale, Calif. PROMcontaining 2048 8-bit words and described in their publication,Signetics Bipolar Memory Data Manual 1982, copyright 1982, which isincorporated herein by reference. Also in the preferred embodiment, signdetector PROM 78 is a type 82S137 PROM, also manufactured by SigneticsCorporation, having 1024 4-bit words and is described in their abovenamed publication.

Decimal add/subtract PROM 84 (also referred to as the decimal ALU) isencoded as shown in Table 3 such that the first four address bits (bitshaving a binary weight of 1, 2, 4 and 8) normally receive operand 1which comes from the output of double multiplexer 83 as signals COP123+through COP120+. These four address bits are also referred to as the Aport of decimal ALU 84 because they receive the first of two decimalnumeric inputs upon which the decimal arithmetic operation is performed.The next four address bits, bits having a binary weight of 16 through128, come from the output of RAM 2 zero multiplexer 90 as signalsCOP223+ through COP220+ which normally will be operand 2 from RAM 2.These four bits are also referred to as the B port of decimal ALU 84because they receive the second of two decimal numeric inputs upon whichthe decimal arithmetic operation is performed. The bit having a binarywait of 256 receives the carry-in signal KCARRY+ and the address bithaving a binary weight of 512 receives signal CIPSUB- from decoder 106which when a binary ZERO indicates that a subtract operation is to beperformed and when a binary ONE indicates that an addition operation isto be performed. The eleventh address bit of PROM 84 having a binaryweight of 1024 is connected to a binary ZERO and therefore results inonly the first 1024 words of PROM 84 being addressed. The invertedfunction (F) input receives a binary ZERO and the two noninverted inputsreceive a binary ONE thereby providing that the 8-bit word outputs Q0through Q7 are continually enabled. The 8-bit word read from decimaladd/subtract PROM 84 as shown in Table 3 contains the 4-bit result ofadding (or subtracting) the 4-bit operand 1 to the 4-bit operand 2 inbits 0 through 3 as signal CANS03+ through CANS00+ and contain the fourindicator bits as signal COPEQ9+ for controlling the equal nineindicator, signal COPEQ0+ for controlling the equal zero indicator,signal CILLEG+ for controlling the illegal nibble indicator and signalCCARRY+ for controlling the carry out indicator.

As discussed above, double multiplexer 84 is used to select the operand1 input to PROM 84 from either RAM 1 in which case signals COP103+through COP100+ are selected, or from the output of RAM 2 multiplexer 89which receives a nibble from RAM 2. If the nibble from RAM 2 multiplexer89 is to be selected as operand 1, then signals COP203+ through COP200+are selected to be gated on to the Q outputs as signals COP123+ throughCOP120+. Double multiplexer 83 is continually enabled by the binary ZEROat its function (F) input and the selection between the zeroth input andthe first input is done under the control of signal CROS38+ at theselect (SEL) input. Signal CROS38+ comes from bit RDDT38 and is a binaryONE if a double output microoperation (CIPDUB) is encoded in themicroinstruction word.

The operand 2 input of decimal add/subtract PROM 84 comes from theoutput of RAM 2 zero multiplexer 90. RAM 2 zero multiplexer selectsbetween the binary ZEROs at the zeroth input or the output of RAM 2nibble multiplexer 89 which is signal COP203+ through COP200+. Theoutputs of RAM 2 zero multiplexer 90 are always enabled by the binaryZERO at the function (F) input. The selection between the binary ZEROand the output of RAM 2 nibble multiplexer to be enabled on to the Qoutputs is done by signal CROS46+ which is inverted by inverter 160 toproduce signal CROS46- at the select (SEL) input of RAM 2 zeromultiplexer 90. Signal CROS46+ is derived from bit RDDT46 of themicroinstruction word which is a binary ONE if the inhibit RAM 2microoperation (CINOP2) is encoded in the microinstruction word.

Result/zone multiplexer 91 is used to select between a zone nibblecontaining a hexadecimal 3 as the zeroth input or the 4-bit decimalresult coming from decimal add/subtract PROM 84 as signals CANS03+through CANS00+ at the first input. Result/zone multiplexer 91 is alwaysenabled by the binary ZERO at its function (F) input. Selection betweenthe zone nibble and the decimal result nibble is done under the controlof signal CWZONE- which at the select (SEL) input which will be a binaryZERO is output by decode PROM 105 when a write zone to RAM 2microoperation (CWZONE) is encoded in the microinstruction word. Theoutput of result/zone multiplexer 91, signal CRES03+ through CRES00+,are fed back as input to nibble multiplexers 92 through 95 thus allowingeither a zone nibble or the decimal result nibble to be written into RAM2 96.

Indicator flip-flops 85-1 through 85-4 are D-type flip-flops and are allclocked by clocking signal KLDFLP- which is connected to their clock (C)input transitioning from the binary ZERO to the binary ONE state. Thistransition of signals KLDFLP- will occur at the end of eachmicroinstruction to load in the status of the indicators as output bydecimal add/subtract PROM 84 when signal PHASEB- at one input of OR gate168 becomes a binary ONE if signal CLDFLP- is a binary ZERO at the otherinput of OR gate 168. Signal CLDFLP- comes from decode PROM 105 and willbe a binary ZERO enabling the clocking of indicator flip-flops 85-1through 85-4 if a load indicator's microoperation (CLDFLP) is present inthe microinstruction. If the load indicator's microoperation is notencoded within the microinstruction, signal CLDFLP- will be a binary ONEand maintain signal KLDFLP- in the binary ONE state thus inhibiting itstransition from the binary ZERO to the binary ONE state which isnecessary in order to clock the indicator flip-flops 85-1 through 85-4.

Both the equal zero indicator flip-flop 85-1 and the equal nineindicator flip-flop 85-2 are arranged in a similar manner such thatduring certain arithmetic operations both flip-flops can be preset sothat their Q output signal will be in the binary ONE state and will bereset if their corresponding output signal from decimal add/subtractPROM 84 becomes a binary ZERO thereafter. For example, equal zeroindicator flip-flop 85-1 is initially set by signal CRESTC- at its set(S) input becoming a binary ZERO which will result in its Q outputsignal LOPEQ0+ becoming a binary ONE at one input to AND gate 165.Thereafter, if the loading of flip-flops is permitted by a loadindicator's microoperation (CLDFLP), equal zero indicator flip-flop 85-1will be clocked at the end of the microinstruction and the output of ANDgate 165 will be determined by the COPEQ0+ signal from decimaladd/subtract PROM 84 which is the other input to AND gate 165. If the8-bit word retrieved from decimal add/subtract PROM 84 contains a binaryONE in the equal zero bit position, when equal zero indicator flip-flop85-1 is clocked the binary ONE at the data (D) input will be clockedinto the flip-flop and the flip-flop will not change state. However, ifthe signal COPEQ0+ is a binary ZERO, the output of AND gate 165, signalKOPEQ0+, will be a binary ZERO and when clocked into flip-flop 85-1 willresult in its output being reset and its output signal LOPEQ0+ becominga binary ZERO. Thereafter, because one input of AND gate 165 is now abinary ZERO, the flip-flop will remain in the reset state independent ofthe binary state of the equal zero bit from decimal add/subtract PROM84. In this manner, the equal zero indicator flip-flop 85-1 and theequal nine flip-flop 85-2 are known as integrating indicator flip-flopsin that the indicator is initially placed in a first state and once itis changed to a second state will remain in the second state independentof the fact that the data input may return to the first state.

Equal nine indicator flip-flop 85-2 is similarly constructed in that itoutputs signal LOPEQ9+ is fed into one input of AND gate 166 whichreceives its other input from decimal add/subtract PROM 84 as signalCOPEQ9+. Both indicator flip-flops 85-1 and 85-2 are initially set bysignal CRESTC- becoming a binary ZERO at their set inputs. Bothflip-flop 85-1 and 85-2 have their reset (R) inputs fixed to receive abinary ONE signal.

The illegal digit (nibble) indicator flip-flop 85-3 is constructed in amanner similar to equal zero and equal nine indicator flip-flops 85-1and 85-2 except that illegal digit indicator flip-flop 85-3 is initiallypreset to the binary ZERO state causing its Q output, signal KILLEG+, tobecome a binary ZERO which is fed back into OR gate 167. Thereafter, ifthe illegal digit indicator bit from decimal add/subtract PROM 84,signal CILLEG+, becomes a binary ONE indicating that the result of theaddition or subtraction has produced an illegal digit because one of thetwo operands contains an illegal digit (see Table 3), then illegal digitindicator flip-flop 85-3 will become set causing its Q output, signalKILLEG+, to assume the binary ONE state and thereafter insure that theoutput of OR gate 167, signal CILDIG+, will remain in the binary ONEstate until such time as the indicator flip-flop is reset. Illegal digitindicator flip-flop 85-3 has its set (S) input fixed to receive a binaryONE and its reset (R) input receives signal CRESTB- from OR gate 162which allows the illegal indicator flip-flop 85-3 to be initially presetto the binary ZERO state.

The carry output indicator flip-flop 85-4 is not an integrating typeflip-flop such that its state will change each time it is clockedindependent of its previous state. Therefore, the data input ofcarry-out indicator flip-flop 85-4 receives its input directly from thecarry bit as output by decimal add/subtract PROM 84 as signal CARRY+. Aswill be seen below, it is sometimes desirable to be able to preset thecarry-out indicator flip-flop 85-4 to the set state thereby causing itsQ output, signal KCARRY+ to be a binary ONE or to be able to preset itto the binary ZERO state and thus presetting its output to the signalKCARRY+ to the binary ZERO state. Carry out indicator flip-flop 85-4 ispreset to the binary ONE state if signal KSETCA- becomes a binary ZEROand is preset to the binary ZERO state if signal CRESTB- at its reset(R) becomes a binary ZERO.

Gates 161 through 164 are used to control the presetting of indicatorflip-flops 85-1 through 85-4. OR gates 161 through 163 each receive asone input a signal PHASEB- which is a clocking signal frommicroprocessor 30. This timing signal when it transitions from thebinary ONE to the binary ZERO state is used to set or reset theindicator flip-flops 85-1 through 85-4 depending upon whether the otherinput of the OR gate is a binary ZERO which is controlled by amicrooperation. For example, OR gate 161 receives signal CRESTX- whichwill be in a binary ZERO state if a reset equal zero and equal nineindicators microoperation (CRESTX) is present in a microinstruction. Ifthis microoperation is present, the ouput of OR gate 161, signalCRESTA-, will become a binary ZERO when the clocking signal PHASEB-comes a binary ZERO and thereby force the output of AND gate 164, signalCRESTC-, to the binary ZERO state and result in the presetting of theequal zero indicator flip-flop 85-1 and the equal nine indicatorflip-flop 85-2 to the binary ONE state. Signal CRESTX- is produced bydecoder 106 as is signal CRESET- at one input of OR gate 163.

Signal CRESET- will be in the binary ZERO state if there is a reset allindicators microoperation (CRESET) in the microinstruction. If signalCRESET- is a binary ZERO at OR gate 163, its output, signal CRESTB-,will become a binary ZERO and resulting in the presetting to the ZEROstate of the illegal digit indicator flip-flop 85-3 and the carry outindicator flip-flop 85-4. Because this signal is also the other input toAND gate 164, it will result in signal CRESTC- becoming a binary ZEROand result in the presetting to the binary ONE state of the equal zeroindicator flip-flop 85-1 and the equal nine indicator flip-flop 85-2.Thus, it can be seen that a resetting of all indicators results in thesetting of flip-flop 85-1 and 85-2 and the resetting of flip-flops 85-3and 85-4.

OR gate 162 produces signal KSETCA- which is connected to the set (S)input of carry out indicator flip-flop 85-4. When this signal becomes abinary ZERO it results in the presetting to the binary ONE state of thecarry out flip-flop thus allowing under microinstruction control thesetting of the carry out indicator to indicate that a carry out has infact occurred. This presetting of the carry out indicator is done by useof the set carry indicator microoperation (CSETCA) which results insignal CSETCA- from decode PROM 105 becoming a binary ZERO.

Sign multiplexer 77 allows the 4 least significant bits of the addressinto sign generator PROM 78 to be selected from either the output ofdouble multiplexer 83 which in turn can have its output selected fromeither a nibble from RAM 1 or a nibble from RAM 2, or alternatively, the4 bits out of sign multiplexer 78 can come from 4 bits within data-inregister 88. Thus, under the selection of the signal CAPACKD- at theselect (SEL) input of sign multiplexer 78, either signals COP120+through COP123+ or signals CBUS04+ through CBUS07+ will be gated on tothe Q output as signals CSIN04+ through CSIN07+. Signal CAPCKD- will bea binary ZERO at the select (SEL) input of sign multiplexer 77 and atthe address bit having a binary weight of 128 of sign generator PROM 78if a packed microoperation (CPACKD) is in the microinstruction asdetermined by decoder 106.

In addition to receiving 4 address bits from sign multiplexer 77, signgenerator 78 receives 3 address bits as signals CPBUS03+ through CBUS01+from the output of data-in register 98. The last address bit of signgenerator PROM 78 is signal COVPUN- which will be a binary ZERO if thereis an overpunch microoperation (COVPUN) in the microinstruction asdetected by decoder 106. The output of sign generator PROM 78 is enabledby the output enable (CE) inputs being fixed to a binary ZERO signal.The 4-bit word read from sign generator PROM 78 results in signalsCOVPUN+, COVPEO+, CSIGNN+ and CSIILL+ which are used to indicate whetherthe sign is an overpunch sign, is an overpunch sign equal to 0, whetherit is a negative sign and whether it is an illegal sign. These four bitsare input into the first input of monitor multiplexer 80. The otherinput of monitor multiplexer 80 receives the output of the decimalindicators 85 such that the output of monitor multiplexer 80 signalsMIBGP0+ through MIBGP3+ will be enabled when the clocking signal PHASEA+at the function (F) input becomes a binary ZERO. The selection betweenthe output of sign generator PROM 78 and the output of decimalindicators 85 is done by signal CROS45+ at the select (SEL) input. Thefirst input from sign generator PROM 78 will be selected if there is asign to microprocessor microoperation (CIPSGN) in the microinstructionwhich will cause the signal CROS45+ to be a binary ONE. The output ofmonitor multiplexer 80 is fed to monitor logic 32 which in turn can beused to control branching within the microprocessor 30 thus allowing thebranching between microinstructions by microprocessor 30 depending uponthe status of decimal indicators 85 or the output of sign generator PROM78. Sign generator PROM 78 is encoded with the necessary data words suchthat its 4-bit output can be used to control branching in themicroinstruction routine which executes the decimal arithmeticoperations. This branching is done early in the execution of a decimaloperation to test the signs of operand 1 and operand 2 before a decimaloperation is performed.

MICROOPERATION DECODING LOGIC DETAILS

FIG. 10D illustrates CIL control area 100 in detail. ROS special controlfield register 101 latches in the special control field bits RDDT35through RDDT47 from the microinstruction word when clocking signalPHASEB-transitions from the binary ZERO to the binary ONE state. Theoutputs of ROS special control field register 101 are enabled by thebinary ZERO at the function (F) input thereof. Signals CROS35+, CROS36+and CROS37- are input to NAND gate 117 to produce signal CIPCOD- at theoutput thereof as an indicator as to whether or not subfield A is abinary 110. Signal CROS37- is produced by inverting signal CROS37+ byinverter 109. If subfield A is a binary 110 indicating that specialcontrol field subfields B, C and D are to be interpreted as commercialinstruction microoperations, signal CIPCOD- will be a binary ZERO andsignal CIPCOD+ at the output of inverter 118 will be a binary one.Special control field subfield B bits which correspond to signal CROS38+through CROS41+ are used as described above to enable various gates andto control the selection of various multiplexers. The special controlfield subfield C bits which correspond to signal CROS42+ through CROS44+are input to decoder 106 to produce signal CPACKD- through CRESET- whichare used as described above. The output of decoder 106 is controlled bysignal CIPCOD- which will cause all the output signals to assume thebinary ONE state if special control field subfield A is not a binary110.

Zero register 102 is connected to receive 8 binary ZEROs as input whichare enabled onto its outputs as signals CNOP48+ through CNOP55+ ifsignal CNOPEN- is a binary ZERO at the function (F) input. The clockinput of zero register 102 is fixed to a binary ONE. Signal CNOPEN- willenable the output if the address presented to ROS 24 is less than 2K.The output of zero register 102, signals CNOP48+ through DNOP55+, arewire-ored together with the output of ROS 24, signals RDDT48+ throughRDDT55+ at wire-or 129 to produce signals KROS48+ through KROS55+.Signals RDDT48+ through RDDT55+ are enabled into wire-or 129 only if theaddress into ROS 24 is greater than 2K.

The data at the input of ROS CIL register 103 is clocked by clockingsignal PHASEB- from microprocessor 30. Signals CROS48+ through CROS51+which correspond to subfield E of the microinstruction word are decodedby use of decode PROM 104 by using the four signals as addressing bits.Signals CROS52+ through CROS55+, which correspond to subfield F of themicroinstruction word, are decoded by use of decode PROM 105 by usingthe four signals as addressing bits. The fifth address bit of decodePROM 104 and decode PROM 105 is tied to a binary ZERO so that the upper16 words in each decode PROM is not used. The output of both decode PROM104 and decode PROM 105 are enabled by the binary ZEROs at theirfunction (F) input. Decode PROM 104 produces signal CLDAD1- throughCTDCT2- and decode PROM 105 produces signals CIPINN- through QLTCTL-,the use of which has been discussed above with reference to FIGS. 10A,10B and 10C. For the most part, the coding of decode PROM 104 and decodePROM 105 is such that only a single output signal will be in the binaryZERO state and all other outputs will be in the binary ONE state exceptfor the few cases in which parallelism is provided by coding the PROMsuch that more than one bit within the 8-bit word is a binary ZERO.These cases are indicated in Tables 7 and 8 above.

DECIMAL SUBTRACT COMMERCIAL SOFTWARE INSTRUCTIONS

In the preferred embodiment, the decimal arithmetic operations that areperformed by CPU 20 are microprogrammed to take advantage of commercialinstruction logic 28 to reduce the execution time of the decimalcommercial software instructions. For example, the decimal multiply anddivide commercial software instructions are sped up by detecting leading(non-significant) zeros in the dividend and divisor and the multiplicandand multiplier, thereby reducing the field lengths which must be usedwhen performing the operations. This leading zero detection is done byusing the ability of RAM 1 81 and RAM 2 96 to be addressed from left toright (i.e. most significant to least significant digit) and by use ofthe decimal equal zero indicator. The decimal add and subtract commerialsoftware instructions are sped up by use of the decimal equal nineindicator to detect cases of oversubtract (i.e., when the sign of thedifference changes) and the ability to feed the decimal adder/subtractorinputs (decimal arithmetic logic unit (ALU) 84 ports A and B) fromeither RAM 1 81 or RAM 2 96 or zeros are used in performing a ten'scomplement. The convert decimal to binary commercial softwareinstruction is sped up by using the ability to address RAM 1 81 for leftto right to strip leading zeros. The convert binary to decimalcommercial software instruction is sped up by using the ability of thedecimal adder/subtractor (decimal ALU 84) to have the same data fed toboth inputs and by presetting the converted value receiving field to azero digit and increasing the receiving field length as necessary (whena carry occurs) as the conversion progresses.

When CPU 20 executes a commercial software instruction, microprocessor30 and commercial instruction logic 28 operate in parallel to performthe necessary microoperations under the control of microinstructionsstored in ROS 24. Microinstruction bits 0 through 47 are used to controlthe operation of microprocessor 30 and microinstruction bits 35 through55 be used to control the operation of commerical instruction logic 28.The use of microinstruction bits 35 through 47 to control microprocessor30 or commercial instruction logic 29 depend on the value of specialcontrol field subfield A. In performing commercial softwareinstructions, microprocessor 30 is used to: read the softwareinstructions from main memory 10, to decode the software instructions,to read the operands from main memory 10, to perform arithmetic,shifting and logical operations on binary data, and write the results ofthe operation back into main memory 10. During execution of commercialsoftware instructions, commercial instruction logic 28 is used toperform logic and shifting operations on decimal and alphanumeric data.

DECIMAL ADDITION AND SUBTRACTION COMMERCIAL SOFTWARE INSTRUCTIONS

The use of commercial instruction logic 28 will now be discussed withreference to the performance of the decimal addition commercial softwareinstruction (DAD) and the decimal subtraction software instruction(DSB). Both the DAD and DSB software instructions are advantageouslyperformed in the preferred embodiment by use of the decimal equal nineindicator and the ability to feed either port of the decimal ALU 84 fromeither RAM 1 81, RAM 2 96 or zeros.

During a DAD or DSB instruction which results in operand 1 (OP 1) beingadded or subtracted from operand 2 (OP 2) with results being stored inthe operand 2. The DAD software instruction uses the absolute valuesubtract routine if the signs of the number to be added are differentand the DSB software instruction uses the absolute value subtractmicroroutine if the signs of the numbers to be subtracted are the same.The equal nine indicator is needed in the absolute subtract routine tohandle the case where the OP 2 field length is less than the OP 1 fieldlength and an oversubtract occurs (i.e., the sign of the differencechanges). This is because the result is calculated in the OP 2 field inRAM 2 and OP 2 is moved from main memory 10 to RAM 2 96 on a word basisand the result which is stored in RAM 2 96 is moved back to main memoryon a word basis. Because OP 2 can start and end on any 4-bit nibbleboundary within the 16-bit words of main memory, it is important thatneighboring nibbles (known as "neighbors") that are not part of OP 2 butwhich are present in the word containing the most significant digit orleading sign of OP 2 and the word containing the least significant digitor trailing sign of OP 2 be preserved during the addition or subtractionso that when the result is written from RAM 2 96 back to main memory 10,the neighbors have not been changed. By preserving the neighbors, theresult can be written back on a word basis without having to read thewords containing the first and last digits, maske in the result and thenwrite back the words to main memory.

In the following discussion, the term "written field" will be used toindicate the field of the result which is as long as the field length ofOP 2 which is used to store the result. The term "unwritten field" willbe used to indicate the left most significant digits in the resulthaving a length equal to the number of digits by which the field lengthof OP 1 exceeds the field length of OP 2. For example, if a DAD isperformed with OP 1=-00142 and OP 2=+68 the result will be -74. In thisexample the written field is 2 digits long (the length of OP 2 withoutthe sign) and the unwritten field is 3 digits long (the length of OP 1minus the length of OP 2, both without signs). This DAD softwareinstruction is performed in CPU 20 by placing OP 1 in RAM 1 81 and OP 2in RAM 2 96 and developing the result in RAM 2 96 from which it iswritten back into the memory locations previously occupied by OP 2.

Example:

    ______________________________________                                        (+68) + (-00142) =  (-74)                                                     + 68    OP 2        in RAM 2                                                  -00142  OP 1        in RAM 1                                                  UUUWW   result,     where:                                                            WW =        written field developed                                                       in RAM 2                                                          UUU =       unwritten field in order                                                      to preserve neighbors in RAM 2                            ______________________________________                                    

In addition to using the equal nine indicator to determine the contentsof the unwritten field, a DAD or DSB software instruction ismicroprogrammed to use the ability to feed the A port of the decimaladder/subtractor 84 from either RAM 1 or RAM 2 and the ability to feedzero's digits into the B port. This is used to perform a ten'scomplement on the written field in those cases where there is anoversubtract.

The following example will describe a DAD software instruction using theexample of OP 1=-00142 and OP 2=+68 which results in a sum of -74. Thesame example would result from a DSB with OP 1=+00142 and OP 2=+68 whichresults in a difference of -74.

For this example, assume the DAD software instruction is at main memorylocation 1000 as follows:

Example DAD software Instruction:

    ______________________________________                                        Memory     Memory                                                             Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                                              Meaning                                               ______________________________________                                        1000       002C         DAD OP code                                           1001       E687         data descriptor 1 (DD1)                                                       word 1                                                1002       1102         data descriptor 1 (DD1)                                                       word 2                                                1003       E207         data descriptor 2 (DD2)                                                       word 1                                                1004       1204         data descriptor 2 (DD2)                                                       word 2                                                ______________________________________                                    

Data descriptors DD1 and DD2 are decoded as follows (see FIG. 9):

DD1:

T=1: Packed decimal.

C1,C2=11: OP 1 starts in nibble 3 position.

C3=1: Trailing sign

L=6: 5 digits and sign.

CAS: OP 1 starts in word addressed by contents of base register 7 plusdisplacement of 1102. If B7 contains the value 1000 hexadecimal, OP 1 islocated at address 2102 hexadecimal

DD2:

T=0: String (unpacked) decimal.

C1=1: OP 2 starts in right byte.

CR,C3=11: Trailing sign.

L=3: 2 digits and sign.

CAS: OP 2 starts in word addressed by contents of base register 7 plusthe displacement of 1204. Since B7 contains 1000 hexadecimal, OP 2 islocated at address 2204 hexadecimal.

OP 1, which is a -00142 with a trailing minus sign, appears in mainmemory as follows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2102           NNN0                                                           2103           0142                                                           2104           DNNN                                                           ______________________________________                                    

Where:

N are neighbor nibbles.

00142 are packed decimal digits.

D is a trailing minus sign.

OP 2, which is +68 with a trailing plus sign, appears in main memory asfollows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2204           NN36                                                           2205           382B                                                           ______________________________________                                    

where:

N are neighbor nibbles.

36 is an unpacked decimal 6 with zone nibble of 3.

38 is an upacked decimal 8 with zone nibble of 3.

2B is an unpacked trailing plus sign.

The execution of the above example DAD commercial software instructionwill now be described with reference to FIG. 11. FIG. 11 is a flow chartof the firmware microroutines used by CPU 20 to execute the DAD and DSBsoftware instruction. The blocks in FIG. 11 which are referred to by thenames next to them, such as DAD-001, show at a gross level the functionsperformed by microprocessor 30 and commercial instruction logic 38 toperform the software instruction. Some of these blocks may represent theexecution of more than one 48 or 56-bit microinstruction, the form ofwhich is shown in FIG. 5.

Before entering the microroutines shown in FIG. 11, which are more orless peculiar to the DAD and DSB commercial software instructions, theCPU 20 examines the first word of the software instruction which isbeing executed to determine the type of operation to be performed. Onceit is determined that it is a decimal arithmetic operation as determinedby looking at the operation code in the first word of the instruction,the CPU 20 then proceeds to decode the address syllable associated withdata descriptor 1 to determine the main memory word address and theposition within the word in which operand 1 begins. This front endprocessing of the software instruction then continues with themicroprocessor branching to the DAD routine to block DAD-000.

When the decimal add routine is entered at block DAD-000, it testswhether this is the first pass in which operand 1 is to be brought intothe CPU or the second pass in which operand 2 is to be brought into theCPU. If it is the first pass, the firmware then branches to instructionDAD-001 which fetches operand 1 into RAM 1 81 one word at a time bybringing it from main memory 10 into the microprocessor 30 and then fromthe microprocessor processor bus 37 into transceivers 97, data-inregister 98 and then into RAM 1 81. This process is performed by firstloading RAM 1 address counter 78 with the address of the first wordwhich is to be used in RAM 1 and the loading of the nibble counter inmibble-out control 76. It should be noted that the words of OP 1 areloaded into RAM 1 by using the low four order bits of the main memoryaddress as the four-bit address which is loaded into RAM 1 addresscounter 75 such that after block DAD-001 is executed, OP 1 is in RAM 1in the following locations with the address counter pointing to word 3in RAM 1 which contains the unit's digit and the nibble counter pointingto nibble 3 which is the position of the unit's digit. At the end ofblock DAD-001, the contents of RAM 1 are as follows:

    ______________________________________                                                     RAM 1                                                            RAM 1        Contents                                                         Location     (Hexadecimal)                                                    ______________________________________                                        2            NNN0                                                             3            0142                                                             4            DNNN                                                             ______________________________________                                         RAM 1 Address Counter (WP1) = 3                                               RAM 1 Nibble Counter (NP1) = 3                                           

Block DAD-001, after setting an indicator to indicate the sign ofoperand 1, then exits to the instruction front end processing routinewhich proceeds to crack data descriptor 2 to determine the address ofwhere OP 2 begins in main memory. After cracking DD2, the front endroutine then branches on the software instruction operating code to aninstruction in block DAD-000 which in turn determines whether this isthe pass 1 of pass 2. In this case it is pass 2 so that block DAD-002 isexecuted.

In block DAD-002 operand 2 is fetched into RAM 2 a word at a time frommain memory, the sign of OP 2 is determined, and the address counter ofRAM 2 address counter 87 and the nibble counter of nibble write control86 are left to point to the unit's position of OP 2 such that at the endof block DAD-002, the contents of RAM 2 are as follows:

    ______________________________________                                                     RAM 2                                                            RAM 2        Contents                                                         Location     (Hexadecimal)                                                    ______________________________________                                         0           NN36                                                             1            382B                                                             ______________________________________                                         RAM 2 address counter (WP2) = 1                                               RAM 2 nibble counter (NP2) = 1                                           

It should be noted that in contrast to RAM 1 where the operand is loadedinto the location which corresponds to the low four order bits of thememory address, the first word of OP 2 is loaded into word 0 of RAM 2and consecutive words are loaded into locations with increasingaddresses.

Block DAD-002 exits to block DAD-003 which determines which operand hasthe shorter field length. This is done by comparing the length of OP 1with OP 2 and determining which is shorter. This determination isnecessary so that the length of the written field can be determined aswell as the length of the unwritten field so that the neighbors withinRAM 2 will not be destroyed if OP 2 is shorter than OP 1. In the presentexample, the length of OP 1 is 5 and the length of OP 2 is 2 (excludingthe signs) such that the shorter field has a length of 2 digits and thislength is the length of Operand 2. Therefore, the written field lengthwill have a length of 2 digits and the unwritten field will have alength of 3 digits. Block DAD-003 then exits to block DAD-004 whichcompares the sign of OP 1 with the sign of OP 2. Block DAD-004determines that OP 1 sign is a minus and that OP 2 is a plus andtherefore exits to block DAD-006 because the signs are different anddoes not exit to block DAD-005 is taken if the signs were the same.Because the signs are different, an absolute value subtract must beperformed and so a call is made to the absolute value subtract routinewhich is entered at block AVS-000.

The absolute value subtract routine performs a subtraction by taking thecontents of RAM 1 from the contents of RAM 2 and placing the result intoRAM 2 a digit at a time starting with the units (the least significant)digit and working from right to left to the most significant digit. Thissubtraction is done in two steps in that first the written field is donein which the result is actually written back into RAM 2 a digit at atime. Then the unwritten field is done and the result of each digitposition is not written into RAM 2 in order to preserve the neighboringnibbles within RAM 2.

The absolute value subtract routine begins in block AVS-000 bypresetting the decimal indicators 85 to an initial value such that thecarry-out (CRO) indicator is set to a binary ZERO, the illegal digit(ILL) indicator is set to a binary ZERO, the equal zero (E0) is set to abinary ONE and the equal nine (E9) is set to a binary ONE. In blockAVS-001 a one digit from RAM 1

In block AVS-001 one decimal digit from RAM 1 is subtracted from onedecimal digit from RAM 2 by decimal ALU 84 with the carry-in coming fromthe carry-out indicator and the resulting digit is stored back into RAM2 and the indicators are stored into decimal indicators 85. BlockAVS-001 is repeated until the length of the shorter operand isexhausted. In this case, because in the example, operand 2 is in string(unpacked) decimal data the zone nibbles within RAM 2 are written backinto RAM 2 by use of the result/zone multiplexer 91 as every othernibble is written into RAM 2 as each decimal digit is processed. In theinstant example, the shorter operand is operand 2, which contains onlytwo unpacked digits. Block AVS-001 is executed two times with thecontents of RAM 2 96 and decimal indicators 85 at the end of the spinthrough the shorter field length (the length of the written field) beingas follows:

    ______________________________________                                        RAM 2     RAM 2                                                               Location  Contents      Indicators                                            (Hexadecimal)                                                                           (Hexadecimal) CRO    ILL    E0  E9                                  ______________________________________                                        0         NN32          0      0      0   0                                   1         362B                                                                ______________________________________                                    

This spinning through the shorter field length of two times bysubtracting first the unit's digit position of RAM 1 from the unit'sdigit position of RAM 2 and storing the result in the unit's digitposition of RAM 2 and then subtracting the 10's position of RAM 1 fromthe 10's position of RAM 2 and storing the result in RAM 2 and thewriting of the zone nibbles into RAM 2 is shown in Table 13 below.

                                      TABLE 13                                    __________________________________________________________________________    Spin Through Shorter Field Length Performing Subtract                                                     DEC ALU   DEC                                     RAM 1          RAM 2         IN OUT   IND                                     STP                                                                              LC CONT                                                                              W1 N1                                                                              LC CONT W2 N2                                                                              A B R CIZN                                                                              CIZN                                    __________________________________________________________________________    1B 2  NNN0                                                                              3  3 0  NN36 1  1 2 8 X XXXX                                                                              0011                                       3  014(2)   1  3(8)2B                                                         4  DNNN                                                                    1A 2  NNN0                                                                              3  2 0  NN36 1  0 X X 6 0000                                                                              0000                                       3  01(4)2   1  (3)62B                                                         4  DNNN                                                                    2B 2  NNN0                                                                              3  2 0  NN36 1  0 X X X XXXX                                                                              0000                                       3  01(4)2   1  (3)62B                                                         4  DNNN                                                                    2A 2  NNN0                                                                              3  2 0  NN3(6)                                                                             0  3 X X X XXXX                                                                              0000                                       3  01(4)2   1  362B                                                           4  DNNN                                                                    3B 2  NNN0                                                                              3  2 0  NN3(6)                                                                             0  3 4 6 X XXXX                                                                              0000                                       3  01(4)2   1  362B                                                           4  DNNN                                                                    3A 2  NNN0                                                                              3  1 0  NN(3)2                                                                             0  2 X X 2 0000                                                                              0000                                       3  0(1)42   1  362B                                                           4  DNNN                                                                    4B 2  NNN0                                                                              3  1 0  NN(3)2                                                                             0  2 X X X XXXX                                                                              0000                                       3  0(1)42   1  362B                                                           4  DNNN                                                                    4A 2  NNN0                                                                              3  1 0  N(N)32                                                                             0  1 X X X XXXX                                                                              0000                                       3  0(1)42   1  362B                                                           4  DNNN                                                                    __________________________________________________________________________

Table 13 illustrates the before and after states of RAM 1 81, RAM 2 96,the output of decimal adder{/fsuracOtor PROM 84 and the status of thedecimal indicators 85 before and after each microinstruction isexecuted. The column labeled "STP" contains the step number with therows labeled with a step number with a "B" suffix containing the statusof the system before the microinstruction is executed and the rows witha step number with an "A" suffix containing the status after themicroinstruction is executed. The columns under the "RAM 1" labelcontain the location, contents, word pointer and nibble pointerassociated with RAM 1 and the column under the "RAM 2" label containingthe location, contents, word pointer and nibble pointer associated withRAM 2. For example, the "LC" column under the "RAM 1" label contains thenumbers 2, 3 and 4 indicating that the corresponding "CONT" columncontains the contents of RAM 1 words addressed by addresses 2, 3 and 4respectively. The "W1" column under the "RAM 1" label contains thecontents of "RAM 1" address counter 75 and the column labeled "N1"contains the contents of the nibble counter in nibble out control 76.The corresponding "LC", "CONT", "W2" and "N2" columns under the "RAM 2"label contain the corresponding data for RAM 2 with the "W2" wordpointer being the address stored in RAM 2 address counter 87 and the"N2" nibble pointer being nibble counter contained in nibble writecontrol 86. The columns labeled under the "DEC ALU" column, contain theinputs and outputs of decimal ALU 84. The "A" and "B" columns under the"IN" label contain the decimal digits input into the A and B ports ofdecimal ALU 84 respectively. Under the "OUT" label, the "R" columncontains the resultant digit output from decimal adder/subtractor PROM84 which is one input to result/zone multiplexer 91 and the columnslabeled "CIZN" contains the four indicator bits output by decimaladder/subtractor PROM 84 which are input into decimal indicators 85. The"C" column corresponding to the carry-out, elsewhere referred to as a"CRO", the "I" column corresponding to the illegal digit signal which iselse sometimes labeled as "ILL", the "Z" column corresponding to theequal zero signal which is elsewhere referred to as "E0" and the "N"column containing the signal indicating the status of the equal ninewhich is elsewhere referred to as "EN". The column labeled "DEC IND"contains the contents of decimal indicators 85 with each individualindicator under the corresponding "CIZN" column.

Step 1B shows the contents of RAM 1 before the microinstruction areexecuted contain the operand 1 with the word and nibble counterspointing to the unit's digit of the number which in the example is thedigit decimal 2. This is indicated in Table 13 by parentheses around thenibble which is point to by the word and nibble counters. Row 1B alsoshows the contents of RAM 2 which indicates that the unit's digit ispointed to by the word and nibble counters such that the decimal 8 iswithin the parentheses. The status of the resultant digit and theindicator bits output by decimal adder/subtractor PROM 84 before themicrooperation is executed are don't care conditions and indicated byX's in Table 13. The decimal indicators have been preset prior to theadding the unit's position to the binary value 0011 as shown.

In step 1, when the unit's digit of operand 1 is added to the unit'sdigit of operand 2, the result is shown in the row labeled step 1a. Step1A shows, in the RAM 1 column, that after the addition the addresscounter and nibble counter have been incremented by one such that thenibble counter now points to the nibble 2 position and the word counterhas remained at word 3 such that the 10's position of the operand 2 isnow pointed to with the parentheses now being around the digit 4.Similarly, after executing the first microinstruction, the word addressand nibble counter of RAM 2 have been incremented to point to the nextnibble, which in this case is field of the unit's position, such thatnibble zero of location 1 is now pointed to which contains the zonenibble of a binary 0011 (hexadecimal 3), as shown in parentheses. Theoutput of the decimal subtract (an absolute value subtract is donebecause the sign of the two numbers to be added together were different)is a decimal 6 as indicated in the "R" column which is the result ofsubtracting 2 from 8. The output of the decimal adder/subtractor PROM 84indicators is a binary 0000 which indicates that there has been nocarry, the digit is a legal digit, the digit is not equal to zero and itis not equal to nine. This results in the updating of the decimalindicators as shown which results in the decimal indicators 85 now beingequal to the binary value 0000. Returning now to the RAM 2 contentcolumn as step 1A, it is seen that the resulting digit output fromdecimal adder/subtractor PROM 84 has been stored back into the nibbleposition which was pointed to at the beginning of the microinstructionsuch that the decimal 6 has been stored in the unit's position ofoperand 2. This storing of the decimal result occurs prior to theupdating of the address and nibble counters such that although at theend of the microinstruction, after its execution, the RAM 2 counterpoint to word 1 nibble 0 the word 1, nibble 1 counter values were usedto store the result before the counters were updated.

The operation shown as step 1 is performed by microinstruction whichcontains a CIPSUB, a CWRES2, a CLDFLP and a CTDALL microoperation withinthe microinstruction. These microinstructions are found in Tables 4through 8 as described above and the operation of which was describedwith respect to the logic shown in FIGS. 10A through 10D. As a briefreview here, the CIPSUB instruction causes the adder/subtractor PROM 84to perform a subtract operation on the two inputs presented to it fromRAM 2 zero multiplexer 90 and double multiplexer 83. In this case, RAM 2zero multiplexer 90 is feeding the output of RAM 2 96 and doublemultiplexer 83 is feeding the output of RAM 1 81. The CWRES2microoperation causes the decimal digit output from the decimaladder/subtractor PROM 84 to be written into the nibble pointed to bynibble write control 86 and RAM 2 address counter 87. The CLDFLPmicrooperation causes the indicators output by decimal adder/subtractorPROM 85 to be clocked into decimal indicators 85. The CTDALLmicrooperation causes the RAM 1 and RAM 2 nibble and word counters to bedecremented after the decimal arithmetic operation has been performed.The microinstruction associated with step 1 also performs other parallelmicrooperations which have not been discussed but, for example, includedecrementing a counter contained within microprocessor 30 to decrementthe length count of the shorter operand field such that the number oftimes through the shorter operand field, where the arithmetic operationis performed on each digit position so that the spinning through theshorter (written) field can be terminated when it is finished.

In step 2 of Table 13, which performs the writing in of the zone nibbleinto the unit's position of operand 2. It being remembered that operand2 in the example case is string (unpacked) data which always has abinary THREE in the zone nibble position of each byte associated witheach decimal digit. The row associated with step 2B shows the contentsof RAM 1 and its associated pointers and the contents of RAM 2 and itsassociated pointers. In the writing of the zone nibble of the unit'sposition, the contents of RAM 1 are ignored and only the contents of RAM2 are used. Before the second microoperation is executed, the wordaddress pointer and nibble pointer of RAM 1 under the W2 and N2 columnspoint to location 1 and nibble 0 respectively such that the zone fieldcontaining a hexadecimal 3 is pointed to as indicated by the 3 beingwithin parentheses in Table 13. The second microinstruction which forcesthe zone field into operand 2 contains a CWZONE and a CTDCT2microoperation as describe in Tables 4 through 8 and the operation ofwhich when the hardware is described in reference to FIGS. 10A through10D above perform the following. The CWZONE microoperation causesresult/zone multiplexer 91 to select the zone bits of binary 0011 as theoutput and result in that zone being written into RAM 2 96 in the nibbleposition pointed to by RAM 2 address counter 87 and nibble write control86. The CTDCT2 microoperation causes the RAM 2 nibble and word countersto be decremented by one after the zone nibble is written into RAM 2 96such that in step 2A it can be seen that the address counter now pointsto word 0 and the nibble counter now points to nibble 3 which containsthe 10's position of operand 2 which is a decimal 6 as indicated by thedigit 6 in the contents column of RAM 2 being within the parentheses. Inthis second microoperation as indicated by rows 2B and 2A, the output ofthe decimal adder/subtractor PROM 84 is a don't care condition and boththe resultant digit and the indicators columns of before and after andthe decimal indicators 85 are not changed as a result of the operation.Steps 3B and 3A show the before and after condition of the execution ofthe third microinstruction which subtracts the 10's position of operand1 in RAM 1 from the 10th position of operand 2 in RAM 2 which results insubtracting 4 decimal from 6 decimal producing a decimal result of 2which is stored in word 0 nibble 3 of RAM 2. The indicators as outputfrom decimal adder/subtractor PROM 84 are again used to update decimalindicators 85. Thus, step 3 is a repeat of step 1 except that the 10'sposition is being manipulated instead of the unit's position. In step 4,which is a repeat of step 2, the zone nibble within the 10's digitposition of operand 2 is written into RAM 2. Thus, at the end of thefourth microinstruction, the pointers and indicators are as shown in row4A.

Returning now to the description of the absolute value add and subtractroutine illustrated in FIG. 11. In block AVS-001 which spins through theshorter length operand field performing the subtraction when it has beendetermined that all digits contained in the shorter field have beensubtracted. Block AVS-001 exits to either block AVS-002 or AVS-004depending upon whether or not the operand 2 field length is shorter thanoperand 1 or if operand 2 is longer or the same length as operand 1. Inthe example case being discussed, operand 2 field length is shorter thanoperand 1 field length so that block AVS-001 exits to block AVS-002.This branching to block AVS-002 is controlled by a flag which waspreviously set in data manipulation area 32 when in block DAD-001 whichdetermines the shorter operand field length. In block AVS-002, themicroinstruction routine restores to indicator to indicate whether ornot the written field contains all zeros. This is done by setting a flagdepending upon the status of the equal zero indicator of decimalindicators 85. This is done by entering decimal indicators 85 viamonitor multiplexer 80 into monitor logic 22 which is then input intomicroprocessor 30 such that the status of the equal zero indicator canbe examined and a flag is set within data manipulation area 32 withinmicroprocessor 30 to remember to status of the zero indicator at the endof the operation on the written field. Block AVS-002 then exits to blockAVS-003 which presets the equal nine and equal zero indicators indecimal indicators 85 in preparation for spinning through the remainderof the longer operand to produce the unwritten field. This presetting ofthe equal nine and equal zero indicators is done by a microinstructioncontaining a CRESTX microoperation.

Block AVS-004 is then entered to spin through the remaining (unwritten)field of operand 1 one digit at a time. Since no more digits existwithin operand 2 to correspond to the hundreds, thousands, etc.positions of operand 1, zero digits are provided by RAM 2 zeromultiplexer 90. The results of the subtraction cannot be written intoRAM 2 because this would result in the destruction of the neighboringnibbles to the left of the most significant digit of operand 2 which arecontained in RAM 2. Therefore, the arithmetic result is not written butthe status of the decimal indicators is continued to be accumulated tobe determined whether the unwritten field is equal to all zeros or allnines, contains an illegal digit or results in a carry-out of the mostsignificant digit. This spinning through of the unwritten field ofoperand 1 is shown in Table 14 which has columns which correspond tothose of Table 13 as described above.

                                      TABLE 14                                    __________________________________________________________________________    Spin Through Unwritten Field Length Performing Subtract                                                   DEC ALU   DEC                                     RAM 1          RAM 2        IN  OUT   IND                                     STP                                                                              LC CONT                                                                              W1 N1                                                                              LC CONT W2 N2                                                                              A B R CIZN                                                                              CIZN                                    __________________________________________________________________________    1B 2  NNN0                                                                              3  1 0  N(N)32                                                                             0  1 1 0 X XXXX                                                                              0011                                       3  0(1)42   1  362B                                                           4  DNNN                                                                    1A 2  NNN0                                                                              3  0 0  N(N)32                                                                             0  1 X X 9 1001                                                                              1001                                       3  (0)142   1  362B                                                           4  DNNN                                                                    2B 2  NNN0                                                                              3  0 0  N(N)32                                                                             0  1 0 0 X XXXX                                                                              1001                                       3  (0)142   1  362B                                                           4  DNNN                                                                    2A 2  NNN(0)                                                                            2  3 0  N(N)32                                                                             0  1 X X 9 1001                                                                              1001                                       3  0142     1  362B                                                           4  DNNN                                                                    3B 2  NNN(0)                                                                            2  3 0  N(N)32                                                                             0  1 0 0 X XXXX                                                                              1001                                       3  0142     1  362B                                                           4  DNNN                                                                    3A 2  NN(N)0                                                                            2  2 0  N(N)32                                                                             0  1 X X 9 1001                                                                              1001                                       3  0142     1  362B                                                           4  DNNN                                                                    __________________________________________________________________________

In Table 14, step 1B shows the decimal indicators 85 have beeninitialized such that the equal zero and equal nine indicators have beenpreset to the binary ONE state and the carry-out and illegal digitindicator have been left in the state they were in at the end of thewritten field subtraction. The microinstruction which corresponds tostep 1 contains a CINOP2, a CIPSUB, a CTDCT1, and a CLDFLPmicrooperation. This microinstruction is repeated for steps 2 and 3also. This microinstruction results in the B port of the decimal ALU 84being fed a decimal zero by selecting the zero input to RAM 2 zeromultiplexer 90. This is done because there is not corresponding digitwithin operand 2 which is stored in RAM 2. The A port of decimal ALU 84is fed the digit pointed to by the word and nibble counters of RAM 1which in step 1 is the hundredths position which contains a decimal 1.

As in the previous steps, the carry input into decimal ALU 84 is fedfrom the carry-out indicator of decimal indicators 85. The result ofsubtracting a 1 from 0 is a decimal 9 as shown in step 1A with aresultant carry-out as indicated in the C column and a resultant equalnine indicator as indicated in the N column. This carry-out and nineresult in the decimal indicator carry bit being set to a binary ONE andthe equal nine bit being set to a binary ONE and the equal zero bitbeing set to the binary ZERO state. This microinstruction also providesthat a subtract operation is done (as opposed to an addition) by thedecimal ALU 84 as specified by the CIPSUB microoperation. The addressand nibble counters associated with RAM 1 81 are decremented after theoperation so that after step 1 the word address counter points to word 3and the nibble counter points to nibble 0. The indicators output by thedecimal ALU 84 are stored in decimal indicators 85 as directly by theCLDFLP microoperation.

Steps 2 and 3 are then repeated for the thousandths and ten-thousandthspositions of operand 1 with corresponding zeros for operand 2 beingsupplied by RAM 2 zero multipliexer 90. Upon the completion of the step3, it can be seen that the word and nibble counters of RAM 1 point tothe first neighboring nibble beyond the most significant digit ofoperand 1 and the decimal indicators contain a binary 1001 whichindicate that there was a carry-out from the last arithmetic operation,that there is no illegal digit within the unwritten field, that theunwritten field is not equal to all zeros and that the unwritten fieldis equal to all nines as indicated by the 3 nines which appear in theresult ("R") column of the rows associated with steps 1A, 2A and 3A.

Block AVS-004 spins through the unwritten operand 1 field until thecounter within microprocessor 30 reaches 0 indicating that the unwrittenfield has been completely processed. Block AVS-004 then exits to themajor branch subtract microoperation which performs a major branchwithin the subtract routine which results in the next microinstructionbeing fetched from ROS 24 as a function of which of 16 conditions existin the four decimal indicators 85 as entered into microprocessor 30 viamonitor multiplexer 80 and monitor logic 22.

Before leaving the absolute value subtract routine, a brief descriptionof block AVS-005 is in order. Block AVS-005 would be entered if thefield length of operand 2 is longer than or the same length asoperand 1. If the operand 1 and operand 2 field lengths are the samelength, nothing remains to be done and block AVS-005 exits to the majorbranch subtract microinstruction which performs the 16-way branchdepending upon the results of the subtraction as indicated by thedecimal indicators 85. If, however, the field length of operand 2 isgreater than the field length of operand 1, the subtraction must becontinued and the results written into RAM 2. In this case, however,because operand 1 is shorter than operand 2, zeros are supplied for thenon-existent leading digits of operand 1 by RAM 1 zero multiplexer 82selecting the zero input such that the A port of decimal ALU 84 would bea decimal 0 and the B port is continued to be fed from RAM 2. It isnecessary to continue to process the operand 2 digits beyond the lengthof the operand because there could be a carry-out from the last digitposition of the subtraction performed by using the most significantdigit of operand 1.

Both blocks AVS-004 and AVS-005 exit by going to the major branchsubtract routine which does a 16-way branch depending upon the 4 decimalindicators from decimal indicators 65. At this point, these decimalindicators contain four binary bits of information which correspond tothe carry-out, illegal digit, the equal zero, and the equal nineindicators all which reflect the result of the last field processedwhich will, in the case of exiting from block AVS-004, be the result ofthe unwritten field, and in the case of exiting from block AVS-005, willbe the result of the written field because there is no unwritten fieldin the case of where the operand 2 field length is greater than theoperand 1 field length. In any case, the major branch subtract routinedoes a 16-way branch depending upon the indicators after the mostsignificant digit has been processed.

The major branch subtract routine is entered at block MBS-000 whichperforms the 16-way major branch after entering decimal indicators 85via monitor multiplexer 80 and monitor logic 22 into microprocessor 30.Block MBS-000 performs a 16-way branch as shown in FIG. 11 which canexit to one of 16 places. The conditions required to enter any one ofthe 16 branch routines are shown by the 4-bit binary number shown at thevarious possible exits. This 4-bit binary number corresponds to thecarry-out, illegal digit, equal zero and equal nine indicators ofdecimal indicators 85. For example, the binary 1000 branch is taken, ifthe carry indicator is a binary ONE indicating there was a carry-out ofthe most significant digit, the illegal indicator is a binary ZEROindicating that there was no illegal digit encountered, the zeroindicator equals a binary ZERO indicating that the field is not allzeros and the equal nine indicator is a binary ZERO indicating that thefield is not all nines.

If the binary 1000 branch is taken, block MBS-001 is entered and jump isperformed to a 10's complement routine. This is one of two cases ofinterest to us to show the use of the equal nine and equal zeroindicators. When the binary 1000 branch is taken, block MBS-001 jumps tothe 10's complement subroutine to perfom a 10's complement on thewritten field because a carry-out from the most significant digit duringa subtract indicates that an oversubtract has been performed and theresult must be 10's complemented. Upon exiting, block MBS-001 performs a2-way branch depending upon whether or not the operand 2 field lengthwas greater than or the same as the operand 1 field length or if theoperand 2 field length was less than the operand 1 field length. Ifoperand 2 was longer or the same length as operand 1, the algebraicresult is not equal to zero and block MBS-002 is entered to set thegreater (G) and the less (L) than commercial instruction indicatorswhich are visible to software instructions. If operand 2 is shorter thanoperand 1, there is an unwritten field which is not equal to all ninesbecause the all nines indicator is not a binary ONE, and therefore, anoverflow (OV) commercial instruction condition has occurred. Therefore,when block MBS-003 is entered, the overflow (OV) commercial instructionindicator is set and upon exiting that block a check is made to seewhether the computer is to trap on overflow and, if so, the trap branchexit is taken to routine OV and, if not, the non-trap branch is taken toroutine SGL.

Block MBS-003 checks whether a trap is to occur if overflow occurs and,if so, traps to the OV routine which initiates a software trap routinewhich will handle the overflow condition. Exiting to the overflow trapsoftware routine results in the result of the decimal arithmeticoperation not being stored in memory. If the trap on overflow has beenmasked out such that trapping does not occur on overflow, SGL blockMBS-003 takes the no-trap exit and goes to the routine which sets thegreater (G) than and less (L) than software visible commercialinstruction indicators. When this routine is processed as will be seenbelow, it results in truncation that saves the least significant digitsof the result by storing the written field in RAM 2 into the memorylocation designated by data descriptor 2.

Before describing the other case of oversubtract in which the indicatorsare equal to a binary 1001 and which begins with the execution of blockMBS-004, the other possible exits from the major branch subtract routinewill be briefly described. If the decimal indicators are equal to abinary 0000, it indicates that no oversubstract has occurred andtherefore the result in RAM 2 is the final result and the routine exitsto the SGL routine which sets the greater than and less than softwareindicators, fixes the sign of the result and writes the result from RAM2 into main memory as will be seen below. If the indicators contain thebinary value X1XX, it indicates that the illegal character indicator hasbeen set indicating that at some point during the arithmetic operation,the decimal ALU 84 encountered an illegal digit on input such that the4-bit nibble contained a value that was greater than decimal 9 (i.e., avalue of from A through F hexadecimal). In the case where an illegaldigit was encountered, a trap occurs to a trap handling routine whichallows the software programmer to write a software routine to handle theillegal operand. In this case, the subtract routine does no clean up andstores nothing into main memory before exiting to the software routinewhich is programmed to handle the case of an illegal digit within one ofthe operands.

If the decimal indicators 85 are a binary 0010, the major branchsubroutine exits to block MBS-008 which does a branch depending upon therelative length of operand 1 and operand 2. If the length of operand 2was greater than or equal to the length of operand 1, the result isequal to all zeros and therefore is not necessary to set either thegreater than or less than indicator so block MBS-008 exits to the FINroutine which completes the processing of the arithmetic operation bysetting the sign in RAM 2 and then writing the result, including thesign, from RAM 2 into the main memory location specified by datadescriptor 2. FIN is a second entry into the SGL routine. If blockMBS-008 takes the branch which indicates that the length of operand 2 isless than the length of operand 1, this indicates that the unwrittenfield is all zeros and a test must be made to see whether the writtenfield is all zeros, therefore block MBS-009 is entered which examinesthe status of the equal zero indicator which was saved at the end of theoperation on the written field. If the equal zero indicator was a binaryONE at the end of the operation on the written field, block MBS-009exits to the FIN routine which means that the result is all zeros andtherefore neither the greater or less than commercial softwareinstruction indicators must be set. If the written field equal zeroindicator was not set at completion of the operation on the writtenfield, block MBS-009 exits to the SGL routine because the result isnon-zero and either the G or L commercial software instructionindicators must be set prior to completing the execution of thecommercial software instruction.

The major branch subtract routine can never take the branch exist whichcorresponds to decimal indicators of a binary 1010 because this is anarithmetically impossible case. This case would indicate that there wasa carry-out of the last (most) significant digit meaning that theabsolute value of operand 2 is less than the absolute value of operand 1but yet the result has the equal zero indicator set to a binary ONEindicating that the result is 0 and this is a mathematicalimpossibility.

If the major branch subtract routine takes the branch corresponding toindicators of a binary 0001, block MBS-010 is entered. This block isentered if the equal nine decimal indicator that is set to the binaryONE state indicating that the result of the last field processed isequal to all nines. Block MBS-010 branches on the relative length ofoperand 1 and operand 2 based on the previous compare of operand lengthswhich was made in block DAD-003. If the length of operand 2 is greaterthan or equal to the length of operand 1 then block MBS-010 branches tothe SGL routine. In this case, the result happens to be equal to allnines. The SGL routine sets the greater and less than indicators beforesetting the sign and then storing the result in main memory. If thelength of operand 2 is less than the length of operand 1, block MBS-010branches to the OV routine because an overflow has occurred. The OVroutine enters block MBS-003 as discussed above which sets the overflowindicator and possibly traps on the overflow condition. This overflowhas occurred because there has been no carry-out of the most significantdigit indicating that there has been no oversubtract so that a 10'scomplement will not be performed. Because the unwritten field is not allzeros the result will not fit into the field length of operand 2 andtherefore an overflow has occurred.

The major branch subtract routine cannot branch to either branchassociated with binary indicators equals to X011 which would mean thatthe all zeros and all nines indicator are both simultaneously set. Thisis an impossible condition to have a field containing both all zeros andall nines and therefore these branch paths are never taken.

Returning now to the other case of interest in which an oversubtract hasoccurred as indicated by the carry-out of the most significant digitmeaning that the sign of the result must be changed and that a 10'scomplement must be performed on the written field. The case where theindicators are equal to a binary 1001 will be described. In this case,the major branch subtract routine block MBS-000 exits to block MBS-004.This block performs a jump to a 10's complete subroutine which performsa 10's complement operation on the written field contained in RAM 2. The10's complement routine TSC is entered and block TSC-000 is executed.

Block TSC-000 resets the RAM 2 address counter and the nibble writecontrol nibble counter to point to the unit's position of the result inRAM 2. A counter in microprocessor 30 is also initialized to the lengthof operand 2 so that it will contain the number of decimal digits(excluding the sign) that are in operand 2 as determined by datadescriptor 2. Block TSC-001 is then entered which spins through thewritten field contained in RAM 2 one digit at a time from the unit'sposition to the most significant digit position until all digits havebeen complemented as determined by the length of operand 2 counter beingdecremented until it reaches 0. This spinning through of the writtenfield to perform the 10's complement is shown in Table 15.

Block TSC-000 before exiting resets the decimal indicators such that thecarry-out and illegal digit indicators are set to binary ZERO and theequal zero and equal nine indicators are set to the binary ONE state.Block TSC-001 spins through the written field contained in RAM 2beginning in the unit's position and working to the most significantdigit performing a 10's complement on each digit. This is done bysubtracting each digit of the result in RAM 2 from zero and storing theresult back into RAM 2. This is done by a microinstruction in blockTSC-001 which contains a CIPSUB, a CIPDUB, a CINOP2, a CWRES2, a CLDFLP,and a CTDCT2 microoperation. These microoperations tell the decimal ALU84 to perform a subtract operation, that the A input of the ALU shouldcome from RAM 2 instead of RAM 1 which is done by selecting the outputof RAM 2 nibble multiplexer 89 to be output by double multiplexer 83,and to provide that the B port of the ALU 84 receives a zero digit fromRAM 2 zero multiplexer 90. In addition, these microoperations providethat the resultant digit out of decimal ALU 84 should be written backinto RAM 2 in the nibble pointed to by RAM 2 address counter 87 and thenibble counter in nibble write control 86. In addition, the decimalindicators 85 are to be updated with the results of the indicators fromdecimal ALU 84 and after the operation is performed, the address andnibble counters in RAM 2 are to be decremented by one to point to thenext more significant digit.

                                      TABLE 15                                    __________________________________________________________________________    Spin Through Written Field Performing Ten's Complement                                                    DEC ALU   DEC                                     RAM 1          RAM 2        IN  OUT   IND                                     STP                                                                              LC CONT                                                                              W1 N1                                                                              LC CONT W2 N2                                                                              A B R CIZN                                                                              CIZN                                    __________________________________________________________________________    1B 2  NN(N)0                                                                            2  2 0  NN32 1  1 6 0 X XXXX                                                                              0011                                       3  0142     1  3(6)2B                                                         4  DNNN                                                                    1A 2  NN(N)0                                                                            2  2 0  NN32 1  0 X X 4 1000                                                                              1000                                       3  0142     1  (3)42B                                                         4  DNNN                                                                    2B 2  NN(N)0                                                                            2  2 0  NN32 1  0 X X X XXXX                                                                              1000                                       3  0142     1  (3)42B                                                         4  DNNN                                                                    2A 2  NN(N)0                                                                            2  2 0  NN3(2)                                                                             0  3 X X X XXXX                                                                              1000                                       3  0142     1  342B                                                           4  DNNN                                                                    3B 2  NN(N)0                                                                            2  2 0  NN3(2)                                                                             0  3 2 0 X XXXX                                                                              1000                                       3  0142     1  342B                                                           4  DNNN                                                                    3A 2  NN(N)0                                                                            2  2 0  NN(3)7                                                                             0  2 X X 7 1000                                                                              1000                                       3  0142     1  342B                                                           4  DNNN                                                                    4B 2  NN(N)0                                                                            2  2 0  NN(3)7                                                                             0  2 X X X XXXX                                                                              1000                                       3  0142     1  342B                                                           4  DNNN                                                                    4A 2  NN(N)0                                                                            2  2 0  N(N)37                                                                             0  1 X X X XXXX                                                                              1000                                       3  0142     1  342B                                                           4  DNNN                                                                    __________________________________________________________________________

Table 15 illustrates the processing of each of the nibbles in thewritten field contained in RAM 2. Step B again shows the contents of RAM1 and RAM 2 prior to the first operation which performs a subtracting ofthe specified digit in RAM 2 from 0 as indicated by the A port beingprovided the decimal 6 from RAM 2 and the B port being provided a 0 fromRAM 2 zero multiplexer 90. After the first step is completed as shown instep 1A, the result of subtracting 6 from 0 is a decimal 4 as shown inthe "R" column which is written into the unit's position in RAM 2 word 1nibble 1 and the indicators have been set to indicate that there hasbeen a carry-out which is really a borrow in this case. The indicatorsfrom decimal ALU 84 are used to update decimal indicators 85 such thatthe carry indicator is set to a binary ONE and equal zero and equal nineindicators have been reset to the binary ZERO state. Step 2 provides forthe writing of the zone field into the string decimal result by use of amicroinstruction containing a CWZONE and a CTDCT2 microoperation. Thismicrooperation results in result/zone multiplexer 91 selecting the zonebits of a binary 0011 to be written into the nibble pointed to by theRAM 2 address counter and the nibble write control nibble counter asshown in step 2A which indicates that the 3 has been written into RAM 2word 1 nibble 0 (no change in this case because the zone nibble wasalready present) and the counters have been updated to point to the 10'sposition in RAM 2.

Step 3 is then performed which performs the 10's complement operation onthe 10's position of the result field by subtracting decimal 2 from adecimal 0 which results in a result of decimal 7 being written into the10's position in RAM 2 as shown in step 3A. The output of the decimalALU indicators is also used to update the decimal indicators 85 which inthis case results in no change. Step 4 is then performed in which thezone nibble is written such that it will contain a hexadecimal 3 and noupdating is done of the decimal indicators 85. After spinning throughthe written field in RAM 2, block TSC-001 exits to block TSC-002.

Block TSC-002 inverts a flag contained in microprocessor 30 which isused to indicate whether operand 2 and a plus or a minus sign initially.This flag is toggled so that the sign of the result will be inverted bythe SGL routine before the result is written back into main memory.Block TSC-002 then returns to the microinstruction following themicroinstruction from which it was called. In this case, the return isto block MBS-004 from which the 10's complement subroutine was called.

Block MBS-004 then exits by branching upon whether operand 2 was longeror the same length as operand 1 in which case the result is not equal tozero and it branches to the set greater than or less than indicatorsroutine SGL. If, however, the operand 2 was shorter than operand 1,block MBS-004 exits to block MBS-006. Block MBS-006 then does a branchdepending upon whether or not the written field after complementingcontains all zeros as indicated by the current status of the equal zeroindicator of decimal indicators 85. Because the unwritten fieldcontained all nines, the overflow condition will only occur if thewritten field is equal to all zeros. Therefore, it the written fieldcontains all zeros, block MBS-006 branches to the OV routine which, asdiscussed earlier, results in the setting of the overflow commercialsoftware instruction indicator and then by performing block MBS-003 andthen testing for if a trap is to be performed on overflow or not. In theparticular case of the example being discussed, the written field is notthe equal to all zeros and block MBS-006 takes the non-zero branch andexits to the SGL routine which sets the greater than or less thancommercial software instruction indicators and writes the result intomain memory.

When the SGL routine is entered, block SGL-000 sets either the G or Lcommercial software instruction indicators depending upon the result ofthe arithmetic operation. If the result is greater than zero, the Gindicator is set to a binary ONE and the L indicator remains reset to abinary ZERO. If the result is less than zero, the L indicator is set toa binary ONE and the G indicator remains in the binary ZERO state. Ifthe result is zero, neither the G nor L indicators are set to a binaryONE (i.e., the L indicator is set to a binary ONE if the sign of theresult is negative and the G indicator is set to a binary ONE if thesign of the result is positive and the equal zero indicator of decimalindicators 85 is not a binary ONE). Block SGL-000 then exits to blockSGL-001 which jumps to a subroutine to set the sign within the result.

In the case of the example being described, the sign of the resultshould be a minus sign because the answer is a -74 decimal and thereforethe sign in RAM 2 must be changed from a plus sign, which is ahexidecimal 2B, to a minus sign, which is a hexadecimal 2D in stringdecimal format. Therefore the trailing sign within the resultant fieldin RAM 1 is changed from a hexadecimal 2B to a hexadecimal 2D such thatthe contents of RAM 2 after the sign position has been set in blockSGL-001 are as follows.

    ______________________________________                                                     RAM 2                                                            RAM 2        Contents                                                         Location     (Hexadecimal)                                                    ______________________________________                                        0            NN37                                                             1            342D                                                             ______________________________________                                    

Block SGL-002 then writes the final result contained in RAM 2 back intomain memory by transferring one word at a time under the control of RAMaddress counter 87 via RAM 2 data register 88 and transceivers 97 a wordof the result back to microprocessor 30 which then writes the word intothe main memory locations pointed to by data descriptor 2. The contentsof memory after block SGL-002 are as follows:

    ______________________________________                                        Memory       Memory                                                           Location     Contents                                                         (Hexadecimal)                                                                              (Hexadecimal)                                                    ______________________________________                                        2204         NN37                                                             2205         342D                                                             string decimal                                                                             74 with trailing minus sign                                      ______________________________________                                    

Thus, at the completion of the decimal addition the contents of memoryin the locations previously occupied by operand 2 will contain a stringdecimal 74 with a trailing minus sign which is the result of adding anoperand 1 of -00142 to an operand 2 of +68. After the complete result iswritten into main memory, block SGL-002 exits to the FETCH routine whichFETCHes the next software instruction from main memory, decodes theoperation codes and begins the processing thereof. The exit to the FETCHroutine completes the processing of the decimal add commercial softwareinstruction.

Before leaving the decimal add commercial software instructiondiscussion, it should be noted that FIG. 11 also contains a flow chartfor the decimal subtract commercial software instruction DSB beginningat block DSB-000. An examination of this flow chart shows that it is thesame as the decimal add instruction except that blocks DSB-005 andDSB-006 are the reverse of blocks DAD-005 and DAD-006 because when asubtract is being performed, if the signs of the operands are differentan absolute value addition must be performed, and if the operands arethe same, an absolute value subtract must be performed. In order tocomplete the discussion of the decimal add and subtract instructions, itshould be noted that the absolute value add routine is not shown in FIG.11 but is similar to the absolute value subtract routine SUB except thatthe blocks corresponding to blocks AVS-001, AVS-004 and AVS-005 performadditions instead of subtractions in the decimal ALU 84. In addition,the blocks corresponding to blocks AVS-004 and AVS-005 in the absolutevalue add routine exit to a major branch addition routine instead of themajor branch on subtraction routine. The major branch addition routine,which is not shown in FIG. 11, is similar to the major branchsubtraction routine shown in FIG. 11 with the exception that in themajor branch addition routine, it is not necessary to do any 10'scomplementing because there is no case in which an oversubtract canoccur since the addition of two numbers having the same sign neverresults in a change of sign.

From the above discussion it can be appreciated that the equal nineindicator and equal zero indicator, which are integrating indicators,are of use in discerning whether the result of an oversubtract in anabsolute value subtract routine is within range when the receiving fieldlength of the result is less than the source operand's field length. Inthe preferred embodiment, where the result is stored into the fieldpreviously occupied by operand 2, this means that if the field length ofoperand 2 is less than the field length of operand 1, the equal nine andequal zero flip-flops can be advantageously employed to determinewhether the result will fit within the field previously occupied byoperand 2. The four cases below will summarize how the equal nineindicator is used.

These cases illustrate than when an absolute subtract is performed, theindicators are initially preset such that the equal nine and equal zeroindicators are binary ONEs and the subtraction is performed startingwith the unit's digits and working towards the most significant digit.The subtraction continues until the written field having a length equalto the length of the shorter operand has been processed. Afterprocessing the written field, the status of the equal zero indicator isremembered for later use. The processing of the rest of the operand thencontinues with the processing of the unwritten field after resetting theequal nine and equal zero indicators before starting processing at theleast significant digit of the unwritten field. The unwritten field isprocessed by zero digits being supplied for the missing digits withinthe shorter operand and the result of the subtracting each digit is notwritten into a field. Instead, the equal zero and equal nine indicatorsare integrated over the unwritten field and together with the carry-outfrom the most significant digit of the longest operand are used toindicate whether the unwritten field is equal to all zeros after anynecessary ten's complementing is performed. In performing the ten'scomplement of the result when an oversubtract occurs (i.e., when thesign of the result must be changed) the ability to to feed either portof the decimal ALU 84 from RAM 2 is used along with the ability to feeda zero into the other port. The case of oversubtract is indicated by acarry out of the most significant digit position. If no carry outoccurs, then the unwritten field must be equal to zero, otherwise theresult will not fit within the receiving field. If a carry out occurs,meaning that an oversubtract has resulted, it means that the writtenfield must be ten's complemented. In this case, when an oversubtract hasoccurred and the written field must be ten's complemented, an unwrittenfield of all nines and a written field ofd all zeros means that theresult will not fit within the receiving field because performing aten's complement will result in a 1 in the first digit of the unwrittenfield and all zeros in the written field (i.e., this is an overflowcondition). If an oversubtract has occurred as indicated by a carry outof the most significant digit, and a ten's complement must be performedon the written field, if the unwritten field is equal to all nines andthe written field is not equal to all zeros, the result will fit with nooverflow. If an oversubtract has occurred, as indicated by a carry outof the most significant digit, and the unwritten field is not equal toall nines, it means that there is an overflow because performing a ten'scomplement on the result would not yield an answer which would fit inthe written field. These conditions are illustrated in the below cases.

Cases illustrating use of equal nine indicator:

    ______________________________________                                        Case 1: DAD (+68)+(-00142)=(-74)                                              ______________________________________                                        11100              Carries out.                                               000                Leading zeros from RAM 2                                                      zero multiplexer 90.                                       68                 OP 2 in RAM 2.                                             -00142             OP 1 in RAM 1.                                             26                 Written field in RAM 2.                                    999                Unwritten field.                                           Carry out of 1 from most significant digit means                              ten's complement necessary on result.                                         Algebraically:                                                                                 100000                                                                       -99926                                                                        -00074                                                        ______________________________________                                         Note:                                                                         Unwritten field of all nines turned into innocuous leading zeros in ten's     complemented result.                                                     

    ______________________________________                                        Case 2: DSB (+68)-(+01142)=(-74)                                              with overflow after truncation                                                ______________________________________                                        11100              Carries out.                                               000                Leading zeros from RAM 2                                                      zero multiplexer 90.                                       68                 OP 2 in RAM 2.                                             -01142             OP 1 in RAM 1.                                             26                 Written field in RAM 2.                                    989                Unwritten field.                                           Carry out of 1 from most significant digit means                              ten's complement necessary on result.                                         Algebraically:                                                                                 100000                                                                       -98926                                                                        -01074                                                        ______________________________________                                         Note:                                                                         Result of -1074 is longer than the 2 digit receiving field. An unwritten      field that is not all nines will not turn into all zeros when ten's           complemented and therefore results in an overflow condition.             

    ______________________________________                                        Case 3: DSB (+68)-(+00042)=(+26)                                              ______________________________________                                        00000              Carries out                                                000                Leading zeros from RAM 2                                                      zero multiplexer 90                                        68                 OP 2 in RAM 2                                              -00042             OP 1 in RAM 1                                              26                 Written field in RAM 2                                     000                Unwritten field                                            ______________________________________                                         Carry out from most significant digit equal to 0 means no need to ten's       complement the result. Equal zero indicator is used to indicate that          unwritten field is all zeros and therefore result fits (no overflow) in       receiving field.                                                         

    ______________________________________                                        Case 4: DAD (+68)+(-00168)=(-00) with overflow                                ______________________________________                                        11100              Carries out.                                               000                Leading zeros from RAM 2                                                      zero multiplexer 90.                                       68                 OP 2 in RAM 2.                                             -00168             OP 1 in RAM 1.                                             00                 Written field in RAM 2.                                    999                Unwritten field.                                           Carry out of 1 from most significant digit means ten's complement             necessary on result.                                                          Algebraically:                                                                                 100000                                                                       -99900                                                                        -00100                                                        ______________________________________                                         Note:                                                                         Result of -100 is longer than the 2 digit receiving field. The unwritten      field of all nines did not turn into innocuous leading zeros. Although th     equal nine indicator was set to 1 at the end of unwritten field, the          written field equal zero indica tor of binary ONE was saved at the end of     written field processing and this condition can be detected as an overflo     condition.                                                               

DECIMAL MULTIPLY COMMERCIAL SOFTWARE INSTRUCTION

The use of commercial instruction logic 28 will now be discussed withreference to the performance of the decimal multiplication softwareinstruction (DML). The DML software instruction is advantageouslyperformed in the preferred embodiment by use of the equal zero indicatorof decimal indicators 85 and the ability to access digits from left toright in RAMs 1 and 2 and also from right to left thus providing thedecimal multiply routine the ability to access the operands from themost significant to the least significant digit, and from the leastsignificant to the most significant digit. Before examining a specificexample of the use of the decimal multiply (DML) software instruction,the overall method used in performing a decimal multiply will beexamined with reference to FIG. 12A which shows a prior art method andFIG. 12B which shows the improved method employed in the preferredembodiment.

In FIG. 12A, the flow chart shows that a multiplication is basicallydone by setting a partial product to zero initially and then workingthrough the multiplier one digit at a time starting with the leastsignificant digit (i.e., the unit's position and then working toward themost significant digits) adding the multiplicand to the partial productthe number of times that correspond to the value of an isolatedmultiplier digit. Once the addition of the multiplicand to the partialproduct has been completed for a given digit, the multiplicand isshifted left one position, which amounts to multiplying it by a decimal10, and the shifted multiplicand is then added to the partial productthe number of times corresponding to the value of the next mostsignificant in the multiplier. This process is continued until all ofthe digits in the multiplier have been used to add the multiplicand tothe partial product.

In the flow chart in FIG. 12A which shows the prior art, themultiplication is begun in the block entitled MUL which is labeled PM00.Block PM00 exits to block PM10 which zeros out the partial product.Block PM10 exits to block PM20 which tests to see whether all digitswithin the multiplier have been processed and if so exit to block PM80which indicates that the multiplication is done. If all digits withinthe multiplier have not been processed, block PM30 is entered and theleast significant non-processed digit within the multiplier is isolated.Block PM30 exits to block PM40 which tests whether the isolated digit ofthe multiplier has been fully processed and if not exits to block PM60which adds the multiplicand to the partial product. As indicated in theflow chart, this addition of the multiplicand to the partial product, isaccomplished by adding each digit of the multiplicand to the partialproduct one digit at a time by performing decimal additions, thisrequires the processing that a loop to be performed a number of timesthere are digits in the multiplicand plus one.

After the multiplicand is added to the partial product, block PM70 isentered and the isolated multiplier digit is decremented. Block PM70then returns to block PM40 which determines whether the decrementedvalue of the isolated multiplier digit is now equal to zero. If thedecremented value of the isolated digit is not equal to zero, block PM40exits to block PM60 which does another addition of the multiplicand tothe partial product which in turn enters block PM70 to decrement theisolated multiplier digit again and this loop from PM70 back to PM40 hasindicated the performed number of times that the equal to the value ofthe multiplier digit.

When block PM40 finally determines that the isolated digit has beendecremented to zero, block PM50 is entered and the multiplicand isshifted one digit position to the left relative to the partial productwhich amounts to multiplying the multiplicand by 10 such that when thenext more significant digit of the multiplier is processed, themultiplicand when it is added to the partial product, will now have avalue of ten times what it had when the previous multiplier digit wasbeing processed. Block PM20 is then entered and a test is made todetermine whether all digits within the multiplier have been processed.If not, the next more significant digit within the multiplier isisolated in block PM30 and PM30 then exits to block PM40.

Block PM40 determines whether the multiplicand has been added to thepartial product the number of times corresponding to the value of theisolated multiplier digit and if not exits to block PM60 to add themultiplicand to the partial product and the isolated multiplier digit isdecremented in block PM70. When block PM40 determines that the isolateddigit has been processed a sufficient number of times such that themultiplicand has been added to the partial product the required numberof times, block PM40 exits to block PM50 which again shifts amultiplicand one digit position to the left relative to the partialproduct which results in again multiplying the multiplicand by 10.

This process of isolating digits within the multiplier and adding themultiplicand to the partial product the number of times equal to valueof the isolated digit continues such that the loop between block PM50back to block PM20 is performed the number times there are digits withinthe multiplier. This process is completed when by block PM20 determinesthat all multiplier digits have been processed so that themultiplication is done and the process is completed in block PM80.

The improved multiplication employed in the preferred embodiment isillustrated in FIG. 12B. In FIG. 12B, the use of the prior art partialproduct method described above with reference to FIG. 12A is basicallyemployed with time improving enhancements performed prior to thebeginning of the process and the clean up enhancements done at the endof the process which results in significant time savings formultiplications performed on operands having leading zeros in themultiplier or the multiplicand.

In FIG 12b, the multiplication begins in the block entitled MUL andlabeled IM00. Block IM00 exits to block IM01 which strips the leadingzeros from the multiplier thereby reducing the effective length of themultiplier. Block IM01 then exits to block IM02 which strips the leadingzeros from the multiplicand and packs the multiplicand such that if itwas unpacked or string data in which each digit occupies a full byte, itis packed such that each digit only occupies a single nibble and thezone nibbles are removed from the multiplicand. In block IM01, thestripping of the leading zeros from the multiplier requires that theloop be performed the number of times that there are leading zeros inthe multiplier. Block IM02 is performed the number of times there aredigits within the multiplicand.

This stripping of the leading zeros from the multiplier and themultiplicand has the effect of reducing the effective length of both themutliplier and the multiplicand. This is done in order to reduce thenumber of times that loops within the multiply routine are performed.This stripping saves time because when the operands are presented to themultiply routine, their length is specified as the length of the entirefield in which the operand resides such that a number having a value of100 occupying a field which is 7 digits in length will have 4 leadingzeros followed by 100. The prior art method will process this number byprocessing each digit within the operand including the four leadingzeros.

After block IM02 has stripped the leading zeros and packed themultiplicand, it exits to block IM10 which begins the multiplicationitself. Blocks IM10, 20, 30, 40, 50, 60, 70 and 80 correspond to blocksPM10, 20, 30, 40, 50, 60, 70 and 80 of the prior art method and work ina similar manner. There is a difference, though, in that by strippingthe leading zeros from the multiplier and multiplicand the number oftimes that loops are performed in the improved method can besignificantly reduced depending upon the number of leading zeros. Forexample, the number of times that IM50 branches back to IM20 is equal tothe number of significant digits in the multiplier as opposed to thenumber of digits in the multiplier which included leading zeros in theprior art method. Similarly, the number of times that the loop withinblock IM60 is performed is equal to the number of significantmultiplicand digits plus 1 as opposed to the prior art method in whichthe loop was performed the number of multiplicand digits plus 1. Becauseblock IM60 contains a loop which is within the bigger loop of blocksIM20 through 50, this reduction in the number of digits which have to beprocessed within the multiplicand can be quite significant.

The stripping of the leading zeros from the multiplier and themultiplicand, thereby effectively reducing the length of the fieldswhich must be processed, is done prior to the beginning of themultiplication itself because if one attempts to determine whether adigit is equal to zero within a multiplier or multiplicand when workingfrom the least significant digit to the most significant digit, it isimpossible to determine whether there is still a non-zero digit to theleft (in a more significant position) of an embedded zero. For example,if the number 1001 is being processed, when the ten's position isprocessed, and the zero is discovered, one cannot simply stop processingat that point because there is a non-zero digit to the left in theone-thousandths position. This non-zero more significant digit is noteasily detected in an efficient manner when working from left to right.

The improved method in FIG. 12B has one other block which is differentfrom that in the method of FIG. 12A in that block IM20 exits to blockIM71 which must then provide leading zeros within the final partialproduct such that the field which receives the results will have thesufficient number of digits. This is done by block IM71 looping from themost significant digit within the partial product until the end of theresultant field length is reached in the most significant digitposition. This loop is performed the number of times there are leadingzeros. Block IM71 then exits to block IM80 which is taken when themultiplication is complete.

From this discussion of the prior art and the improved method, it shouldbe appreciated that the improved method has a little overhead at thebeginning of the multiplication to strip of the leading zeros and alittle overhead at the end of the multiplication operation to provideleading zeros in the final product but that this additional overhead ismore than compensated for by the great reduction in the number of timesnested loops within the multiplier routine must be performed.

The stripping of leading zeros in the multiplier multiplicant can becomesignificant when the number of significant digits within the operandfield is relatively small as can be the case quite often in softwareprograms. This is the case because when a software program is written,the operand field lengths are specified such that it is sufficient longto accommodate the maximum length operand such that a field may berequired to hold. For example, an operand field may be specified ascontaining 16 digits when, in fact, for a large portion of the time thenumber may never exceed 1000 such that the operands will usually have 13or more leading zeros within them. This happens because it is easier forthe software to use fixed length fields for the operands rather than tocontinually adjust the operand field lengths as a function of the valueof the variable stored in that field at any given moment. For example,if an operand value may go to 99,999,999, rather than to attempt tocontinually adjust the field length of the operand which may range froma value of 0 up to the maximum number and usually has a value of lessthan 999, the operand is specified to have 8 decimal digits.

The implementation of the DML software instruction will now be discussedwith reference to the detailed flow charts of FIG. 13 which show itsimplementation in a preferred embodiment. In the DML softwareinstruction, operand 1 is the multiplier which is used to multiplyoperand 2 which is the multiplicand and the product is stored in thefield previously occupied by operand 1.

The DML software instruction will now be explored taking as a specificexample as follows:

Example:

    (-000,000,002,403)*(+000,305)=(-000,000,732,915)

In this example, a multiplicand of -2403 which is operand 2 or OP 2 ismultiplied by a multiplier of +305 which is operand 1 or OP 1 to yield aproduct of -732915 which is stored in the field previously occupied byOP 2. In this example, the multiplicand has 8 leading zeros and themultiplier has 6 leading zeros which will be stripped away before theactual multiplication is performed. Once the final product iscalculated, leading zeros will be added to the product before it isstored into main memory.

This stripping of the leading zeros from the multiplicand and themultiplier before calculating the product and the addition of leadingzeros to the product at the end greatly reduces the number of stepsrequired as was discussed above. In addition, the preferred embodimentalso packs unpacked decimal data such that if string data is presented,in which case each digit of an operand requires a single byte in whichthe left nibble contains a zone field of binary 0011 and the rightnibble contains a decimal value, it is packed to eliminate zone nibblesso that the decimal digits may be consecutively addressed in the workingarea of the commercial instruction logic. This packing of the operandsaves having to skip the zone nibbles and results in some operationsbeing performed in a single microinstruction as opposed to requiring twomicroinstructions which would be necessary in order to skip the zonenipples. The above example values will now be discussed in more detailwith reference to the detail flow charts of the decimal multiply method.

For this example, assume the DML software instruction is at main memorylocation 1000 as follows:

Example DML Software Instruction

    ______________________________________                                        Memory    Memory                                                              Location  Contents                                                            (Hexadecimal)                                                                           (Hexadecimal) Meaning                                               ______________________________________                                        1000      0029          DML op code                                           1001      E707          data descriptor 1 (DD1)                                                       word 1                                                1002      1102          data descriptor 1 (DD1)                                                       word 2                                                1003      6B07          data descriptor 2 (DD2)                                                       word 1                                                1004      1204          data descriptor 2 (DD2)                                                       word 2                                                ______________________________________                                    

Data descriptors DD1 and DD2 are decoded as follows (see FIG. 9):

DD1:

T=0: String (unpacked) decimal.

C1=1: OP 1 starts in right byte.

C2,C3=11: Trailing sign

L=8: 7 digits and sign.

CAS: OP 1 starts in word addressed by contents of base register 7 plusdisplacement of 1102. If B7 contains the value 1000 hexadecimal, OP 1 islocated at address 2102 hexadecimal.

DD2:

T=0: String (unpacked) decimal.

C1=0: OP 2 starts in left byte.

C2,C3=11: Trailing sign.

L=3: 2 digits and sign.

CAS: OP 2 starts in word addressed by contents of base register 7 plusthe displacement of 1204. Since B7 contains 1000 hexadecimal, OP 2 islocated at address 2204 hexadecimal. OP 1, which is a +000305 with atrailing plus sign, appears in main memory as follows:

    ______________________________________                                        Memory         Memory                                                         Location       Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2102           NN30                                                           2103           3030                                                           2104           3330                                                           2105           352B                                                           ______________________________________                                    

wherein:

N are neighbor nibbles.

30, 33 and with 35 are unpacked decimal digits and with zone fields andcorresponding to 0, 3 and 5 decimal.

2B is an unpacked trailing plus sign.

OP 2, which is -2403 with a trailing minus sign, appears in main memoryas follows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2204           3030                                                           2205           3030                                                           2206           3030                                                           2207           3030                                                           2208           3234                                                           2209           3033                                                           220A           2DNN                                                           ______________________________________                                    

where:

N are neighbor nibbles which must be preserved when product is stored.

30, 32, 34 and 33 are unpacked decimal digits with zone fieldscorresponding to 0, 2, 4 and 3 decimal.

2D is an unpacked trailing minus sign.

The execution of the above example DML commercial software instructionwill now be described with reference to FIG. 13. FIG. 13 is a flow chartof the firmware microroutines used by CPU 20 to execute a DML sofwareinstruction. The blocks in FIG. 13 which are referred to by the namesnext to them, such as DML-001, show at a gross level the functionsperformed by microprocessor 30 and commercial instruction logic 28 toperform the software instruction. Some of these blocks may represent theexecution of more than one 48 or 56-bit microinstruction, the form ofwhich is shown in FIG. 5. Before entering the microroutines shown inFIG. 13, which are peculiar to the DML commercial software instructions,the CPU 20 examines the first word of the software instruction which isbeing executed to determine the type of operation to be performed. Onceit is determined that it is a decimal arithmetic operation as determinedby looking at the operation code in the first word of the instruction,the CPU 20 then proceeds to decode the address syllable associated withdata descriptor 1 to determine the main memory word address and theposition within the word in which operand 1 begins. This front endprocessing of the software instruction then continues with themicroprocessor branching to the DML routine at block DML-000.

When the decimal multiply routine is entered at DML-000, it determineswhether this is a first pass in which operand 1 is to be brought intothe CPU or the second pass in which operand 2 is to be brought into theCPU. If it is the first pass, block DML-000 branches to block DML-001which fetches operand 1 into RAM 1 one word at a time by bringing itfrom main memory into the microprocessor, from the microprocessorprocessor bus 37 into transceivers 97, data-in register 98 and then intoRAM 1 81. This process is performed by first loading RAM 1 addresscounter 75 with the address of the first word which is to be used in RAM1 and then the loading of the nibble counter in nibble-out control 76.It should be noted that the words of OP 1 are loaded into RAM 1 by usingthe low four-bit order bits of the main memory address as the four-bitaddress which is loaded into RAM 1 address counter 75 such that at theend of block DML-001, the contents of RAM 1 are as follows:

    ______________________________________                                                     RAM 1                                                            RAM 1        Contents                                                         Location     (Hexadecimal)                                                    ______________________________________                                        2            NN30                                                             3            3030                                                             4            3330                                                             5            352B                                                             ______________________________________                                    

Block DML-001 then exits to block DML-002 which then strips off leadingzeros of OP 1 as it moves a copy of OP 1 into segment 0 of RAM 2. Duringa multiply operation, RAM 2 is broken in eight 32-word segments suchthat segment 0 occupies address 0-1F (hexadecimal), segment 1 occupiesaddressed 20-3F (hexadecimal) and segment 2 occupies address 40-5F(hexadecimal), etc. This move and strip of leading zero operation isperformed by a subroutine that takes advantage of the ability of the RAM1 and RAM 2 to be addressed from left to right (i.e., from the mostsignificant digit to the least significant digit).

The move and strip subroutine is entered with the address and nibblecounter of RAM 1 pointing to the most significant digit (nibble) of OP 1in RAM 1 and with the word counter of RAM 2 pointing to word 0 insegment 0 (i.e., location 0) and the nibble counter pointing to nibble3. The routine is also entered with a a word of all binary zeros loadedinto data-in register 98 and the decimal indicators 85 preset such thatthe equal zero indicator is a binary ONE. The routine then writes oneword of all zeros from data in register 98 into the word pointed to bythe address counter of RAM 2 by a CWROP2 microoperation. The routinetakes the nibble pointed to in RAM 1 and runs it through decimal ALU 84and writes it into the right (3rd) nibble of the word pointed to in RAM2 by use of a microinstruction containing a CWRES2, a CLDFLP, a CRUCT1and a CINOP2 microoperation. This microinstruction also loads thedecimal indicators to indicate if the digit just moved to RAM 2 was anibble containing a decimal 0 digit and it also increments the nibblecenter of RAM 1 to point to the next nibble which results in the wordcounter of RAM 1 being incremented if the nibble counter incrementsthrough 3. The routine then checks if OP1 is string data and if so itincrements the nibble counter of RAM 1 one more time to skip over thezone nibble of the next least significant digit in RAM 1. The routinethen tests to see if the equal zero decimal indicator is still a binaryZERO which means that the digit moved from RAM 1 to RAM 2 was a leadingzero digit. If the moved digit was a non-zero digit, the routineincrements the address counter of RAM 2 so that it will point to thenext word by use of a CIAD01 microoperation and then checks to see ifall of OP 1 has been moved to RAM 2. If some of OP 1 remains to bemoved, the routine returns to the beginning and writes a word of zerosinto RAM 2 and then moves the next digit of OP 1 from RAM 1 into RAM 2.If the moved digit of OP 1 was a zero digit such that the equal zeroindictor is still a binary ONE, the routine test if all of OP 1 has beenmoved and if not it goes back and moves the next digital of OP 1 fromRAM 1 to RAM 2 without having to zero out the word pointed to by the RAM2 address counter because the counter is still pointing to the wordwhich has been previously zeroed out since a non-zero digit has not yetbeen encountered. The strip and move routine makes use of theintegrating nature of the equal zero decimal indicator in that once anon-zero digit is encountered, the address counter is incremented by oneas each digit is moved even if it happens to be an embedded zero as isfound in the number 1203.

When the move and strip routine finishes the move, it returns to blockDML-002 which tests the status of the illegal digit indicator todetermine if an illegal digit is present in OP 1. The illegal digitindicator was preset to a binary ZERO prior to calling the move routineand since it is also an integrating indicator, it states at the end ofthe move will indicate if any illegal (non 0-9 digit) was encounteredduring the move. If an illegal digit was encountered block DML-002 exitsto routine IC otherwise it exits to block DML-003.

At the completion of block DML-002 the contents of segment 0 of RAM 2will have the significant digit of OP 1 in it as follows:

    ______________________________________                                                     RAM 1                                                            RAM 1        Contents                                                         Location     (Hexadecimal)                                                    ______________________________________                                        0            0003                                                             1            0000                                                             2            0005                                                             ______________________________________                                    

This format, in which RAM 2 contains only one digit of OP 1 (themultiplier) per 16-bit word right justified, makes the digits easilyaccessible for entry and use in microprocessor 30 as counters in themultiplier loops later on. Block DML-003 then computes the number ofsignificant digits in the OP 1 which is equal to the number of wordsoccupied in RAM 2 segment 0 by the multiplier after the leading zeroshave been stripped. This number of significant digits in the multiplieris used as a counter in the multiplication loop.

Block DMA-003 then exits to the instruction front end processing routinewhich proceeds to crack data descriptor 2 to determine the main memoryaddress of where OP 2 begins in main memory. After cracking DD2, thefront end routine then branches on the software instruction operationcode in block DML-000 which in turn determines whether this is the pass1 of pass 2. In this case it is pass 2 so that the microinstructionsassociated with block DML-004 are executed.

In block DML-004 operand 2 is brought into both RAM 1 and segment 1 ofRAM 2 a word at a time from main memory and the sign of OP 2 isdetermined. The copy of OP 2 in segment 1 of RAM 2 is used primarily topreserve the neighbor nibbles so that when the product is written backinto main memory in the field previously occupied by OP 2 the neighborsare not destroyed. At the end of block DML-004, the contents of RAM 1and segment 1 of RAM 2 are as follows:

    ______________________________________                                        RAM 1          RAM 1                                                          Location       Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                         4             3030                                                            5             3030                                                            6             3030                                                            7             3030                                                            8             3234                                                            9             3033                                                           A              2DNN                                                           ______________________________________                                        RAM 2          RAM 2                                                          Location       Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        10             3030        Segment 1                                          11             3030                                                           12             3030                                                           13             3030                                                           14             3234                                                           15             3033                                                           16             2CNN                                                           ______________________________________                                    

It should be noted that in contrast to RAM 1 where the operand is loadedinto the location which corresponds to the low four order bits of thememory address, the first word of OP 2 is loaded into word 0 of segment1 of RAM 2 and consecutive words are loaded into locations withincreasing addresses.

In block DML-005 the sign of the result is calculated by comparing thesign of operand 1 with the sign of operand 2. The resultant sign ispositive if the signs of the two operands are the same, negative if thesigns of the operands are not the same.

In block DML-006, operand 2 is packed into segment 2 of RAM 2. Thispacking is done by calling in a subroutine which takes the copy ofoperand 2, which is in RAM 1, and packs it into RAM 2 segment 2. Thispacking operation also strips off leading zeros. Before performing thepacked routine, a test is made in block DML-006 to determine whetheroperand 1 is equal to 0 and, if so, a branch is taken to the resultroutine (RES). The strip and pack routine used in block DML-006 isinitially called with the address counter and nibble counter of RAM 1set to point to the most significant digit of the operand 2 which isstored in RAM 1 and with the address counter and nibble counter of RAM 2to point to word 0 of segment 2 and nibble zero within word 0. Theroutine then performs a microinstruction which contains a CWRES2, aCLDFLP, a CTUCT1, and a CINOP2 microoperation which results in thenibble within RAM 1 going through decimal ALU 84 through result/zonemultiplexer 91 and into the nibble pointed to by the RAM 2 address andnibble counters. The RAM 1 pointers are then updated to point to thenext nibble within RAM 2 to the right of the nibble which was just movedand the RAM 2 counters are left unchanged. A test is then made todetermine whether or not operand 2 is in the string decimal format(i.e., if it contains zone nibbles) and if so the RAM 1 nibble counteris incremented by one to skip over the zone field and the RAM 1 addresscounter is incremented by one if the RAM 1 nibble counter incrementsthrough 3. A test is then made on the equal zero indicator to seewhether all of the digits that have been moved through decimal ALU 84have been equal to zero digits. If a non-zero digit has been movedthrough the decimal ALU 84, the nibble pointer in RAM 1 is incrementedby one to point to the next nibble to the right. Again, if the nibblecounter increments through 3, the corresponding address counter isincremented by one. Here again, use is being made of the integratingnature of the equal zero indicator such that leading zeros will bestripped from the operand when the operand is being packed but, once anon-zero digit is encountered, from then on all digits are moved suchthat even embedded zeros will be moved. This incrementing of the RAM 2pointers when a non-zero digit is moved is done by a microinstructioncontaining a CTUCT2 microoperation. This process is then repeated untilthe complete field length of operand 2 has been processed.

Upon completion of the move of operand 2 from RAM 1 and the stripping ofthe leading zeros and packing it in RAM 2 a test is then made todetermine whether operand 2 is equal to zero by testing the equal zeroindicator. If the equal zero indicator is still set indicating that alldigits were zero, a branch is taken to the result routine (RES) becausethe answer is now known because one of the operands is equal to zero. Afurther test is made to determine whether operand 2 contained an illegalcharacter for a digit and, if so, a branch is taken to illegal character(Iroutine IC). Upon the completion of block DML-006, the contents ofsegment 2 of RAM 0 are as follows:

    ______________________________________                                        RAM 2      RAM 2                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                        20         2403         Segment 2                                                                     OP 2 stripped and packed.                             ______________________________________                                    

Block DML-006 also calculates the number of significant digits inoperand 2 which is equal to the number of digits of operand 2 stored inRAM 2 segment 2.

Block DML-007 is then entered to move the packed copy of operand 2 fromRAM 2 to RAM 1. This moved is performed a word at a time by reading aword from RAM 2 into RAM 2 data-in register 88 and back into data-inregister 98 and from there into RAM 1. This move is performed by amicroinstruction containing a CWROP1, a CIAD01, and a CIAD02microoperation which results in the writing of RAM 1 and theincrementing of the address counters of RAM 1 and RAM 2 after the readand write have been performed. Before the move loop is initiated, boththe RAM 1 address counter and the RAM 2 address counter were loaded withaddresses to point to the first word of RAM 1 (word 0) and RAM 2 addresscounter was loaded to point to the first word of segment 2 (word 20).After the packed version of operand 2 in RAM 2 segment 2 has been movedto RAM 1, the contents of RAM 1 are as follows:

    ______________________________________                                        RAM 1      RAM 1                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                        0          203          OP 2 stripped and packed.                             ______________________________________                                    

Block DML-008 is then entered which zeroes out the partial product fieldprior to initializing the multiplication loops. The partial product isstored in RAM 2 segment 2. This zeroing out of segment 2 of RAM 2 isperformed by initializing the RAM 2 address counter to point to thefirst word of segment 2 (i.e., word 20) and then loading a word of allzeros into data-in register 98 and performing a microinstructioncontaining a CWROP2 and a CIAD02 microoperation which results in thewriting of a word into RAM 2 after which the RAM 2 address counter isincremented by one. This loop is continued until 8 words of segment 2 inRAM 2 have been written into. At the end of block DML-008 the contentsof RAM 1 and RAM 2 are as follows:

    ______________________________________                                        RAM 1     RAM 1                                                               Location  Contents                                                            (Hexadecimal)                                                                           (Hexadecimal)                                                       ______________________________________                                         0        2403           Packed version of                                     1        XXXX           OP 2 multiplicand                                    ______________________________________                                        RAM 2     RAM 2                                                               Location  Contents                                                            (Hexadecimal)                                                                           (Hexadecimal)                                                       ______________________________________                                         0        0003           Segment 0                                             1        0000           Significant digits                                    2        0005           OP 1 multiplier                                       3        XXXX           1 digit/word.                                        10        3030           Segment 1                                            11        3030           Main memory image                                    12        3030           of OP 2 multiplicand.                                13        3030           Preserved or neighbor                                14        3234           nibbles. Product will                                15        3033           be unpacked into this                                16        2CNN           area and filled with                                 17        XXXX           leading zeros.                                       20        0000           Segment 2                                            21        0000           Partial product                                      --        --             of 32 packed zeros                                   28        0000           ready for multiplica-                                29        XXXX           tion.                                                ______________________________________                                    

As can be seen above, the partial product area of segment 2 has beenpacked with 32 zeros which only requires the use of 8 words. The 32zeros are used because this is the maximum length of any decimal numberin the preferred embodiment.

The multiply loop itself begins at the entry point labeled MULT at blockDML-009. Block DML-009 gets the current multiplier digit from RAM 2segment 0 and stores it into a register in microprocessor 30 where itcan be decremented down each time the multiplicand is added to thepartial product. Initially, because the multiplication is performed byusing the multiplier digits from right to left, that is, themultiplication begins using the least significant (the unit's position)digit, the first time through block DML-009 loads word 2 from word 2 RAM2 segment 2 into microprocessor 30.

Block DML-009 then exits to block DML-010 which branches depending uponthe value of the current multiplier digit which is being worked on. Thiscurrent multiplier digit value is decremented down by one each time themultiplicand is added to the partial product. As long as the decrementedvalue of the multiplier digit is not equal to zero, block DML-010 exitsto block DML-011 and begins the addition of the multiplicand to thepartial product. When the decremented value of the current multiplierdigit is equal to zero, block DML-010 exits to block DML-017. Initially,because the unit's position of the multiplier in the example contains amultiplier digit of 5, it is not equal to zero and block DML-010branches to block DML-011.

In block DML-011 the multiplicand is added to the partial product. To dothis, the multiplicand which is stored as the packed version of operand2 in RAM 1 is added to the partial product which is stored in segment 2of RAM 2. This addition is done by adding each digit of the multiplicandin RAM 1 to its corresponding digit within the partial product in RAM 2with the addition being performed by adding the unit's position of themultiplicand to the adjusted unit's position in the partial product andthe adding the ten's digit in the multiplicand to the adjusted ten'sposition in the partial product. This addition loop adding digit bydigit of multiplicand to partial product is done the number of timesthat there are significant digits in the multiplicand and therefore thestripping of leading zeros done initially from the multiplicand reducesthe number of times that this loop must be done. The initial unit'sposition within the partial product of RAM 2 segment 2 which isinitialized to all zeros is initially set at the nibble whichcorresponds to the Lth nibble in segment 2 of RAM 2 where L is equal tothe number of digits not including the sign in operand 2 because theoperand 2 field will be used to hold the final product. In the example,there are 12 digits in operand 2 and therefore the 12th nibble in RAM 2segment 2 is the unit's position in the partial product such that theunit's position is contained in word 22 nibble 3. Thus, before the firstaddition is done within block DML-011, the pointers in RAM 1 and RAM 2are as shown below in which the address counter of RAM 1 points to word0 and the nibble counter of RAM 1 points to nibble 3 such that theunit's digit of 3 will be processed and the pointers to the partialproduct in RAM 2 are set to point to the unit's position in RAM 2segment 2 such that the word counter 2 points to word 22 and the nibblecounter points to nibble 3.

    ______________________________________                                        RAM 1         RAM 1                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                         0            2403                                                                                     WP1=0                                                                         NP1=3                                                ______________________________________                                        RAM 2         RAM 2                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                        20            0000                                                            21            0000                                                            22            0000                                                                                     WP2=22                                                                        NP1=3                                                ______________________________________                                    

This addition is accomplished by block DML-011 executing amicroinstruction containing a CWRES2, a CTDCT1, a CTDCT2, and a CDLFLPmicrooperation such that one nibble from RAM 1 is added to one nibblefrom RAM 2 and written back into RAM 2 with the decimal indicators setto indicate the result of the addition. At the end of the additionmicroinstruction, the counters in RAM 1 are decremented as are thecounters in RAM 2 to point to the next most significant digit within themultiplicand and partial product respectively. This loop is then donethe number of times corresponding to the number of significant digits inthe multiplicand so that each digit within the multiplicand is added tothe partial product. In the example, this results in the four digits ofthe multiplicand being added to the partial product such that at the endof the loop, the partial product will contain the following:

    ______________________________________                                        RAM 2         RAM 2                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                        20            0000        Segment 2                                           21            0000                                                            22            2403                                                                                      WP2=2                                                                         NP2=2                                               ______________________________________                                    

Block DML-011 then exits to block DML-012 which determines whether thereis sufficient room to expand the partial product value. This testdetermines whether the adjusted unit's position in the partial productis equal to the adjusted number of significant digits being used in themultiplicand. If the adjusted unit's position is equal to the adjustednumber of significant digits in the multiplicand, then there is no roomto expand the partial product to accommodate any possible carry-out ofthe addition of the most significant in the multiplicand to the mostsignificant digit in the partial product. If there is no room to expandthe partial product, then DML-012 exits to block DML-013 whichdetermines whether there is a need to expand the partial product. Thisis done by determining whether there is a carry-out of the addition ofthe last digit of the multiplicand with the last digit of the partialproduct (i.e., the most significant digit of each). If there is nocarry-out (the carry-out indicator of decimal indicators 85 is equal to0) there is no overflow and block DML-016 is entered. If there was acarry-out of the most significant digit, then an overflow conditionexists and block DML-014 sets an overflow flag before exiting to blockDML-016. If block DML-012 determines that there is room to expand thepartial product, then the one more digit addition is performed by usinga microinstruction containing a CINOP1 and a CWRES2 microoperation whichresults in the next digit within the partial product being added to azero from RAM 1 zero multiplexer 82 with the carry-out from the previousdigit being added as the carry-in in this addition. Block DML-015 thenexits to block DML-016 which decrements the current multiplier digitbefore returning to block DML-010 for the next multiplier digit.

Block DML-010 then tests the decremented value of the current multiplierand depending upon whether it is equal to zero or non-zero exits toblock DML-011 or DML-017. Because the unit's position of the multipliercontains a decimal 5, the branch to block DML-011 would be taken 5 timesadding the multiplicand into the partial product 5 times at the end ofwhich the partial product in RAM 2 segment 2 will be as follows:

    ______________________________________                                        RAM 2         RAM 2                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                        20            0000        Segment 2                                           21            0001                                                            22            2015                                                            ______________________________________                                    

This is the equivalent of 5 times the multiplicand. When the currentmultiplier digit is exhausted in block DML-010, it exits to blockDML-017. The pointer to the current multiplier digit is decremented topoint to the next more significant digit in the multiplier which, thefirst time this block is entered, results in the pointer being changedfrom the unit's position to the ten's position. Block DML-017 then exitsby testing to see whether there are more multiplier digits to beprocessed and, if not, exits to block DML-022.

If there are more digits to be processed in the multiplier, blockDML-017 exits to block DML-018 which shifts the multiplicand left onedigit (i.e., equivalent to multiplying it by 10) relative to the partialproduct. This shifting of the multiplicand relative to the partialproduct is actually done by shifting the pointer to the unit's positionin the partial product to point to the next digit to the left such thatthe unit's position in the multiplicand will now be added to theadjusted unit's position in the partial product. The first time blockDML-018 is executed, this amounts to changing the pointers to thepartial product in RAM 2 segment 2 such that the unit's position is nolonger considered to be in word 22 nibble 3, but is now considered to bein word 22 nibble 2, such that the unit's position of the multiplicandwill now be added to the ten's position and the partial product. BlockDML-018 then exits to block DML-020 if this shifting of the startingposition in the partial product does not result in the starting positionbeing shifted out of the most significant digit of the partial product(i.e., out of word 20 nible 0). If the shift results in the startingposition in the partial product being shifted out of the mostsignificant digit of the partial product, block DML-019 is entered andthen overflow condition is flagged. However, if the starting position inthe partial product has not been shifted out of the most significantdigit of the partial product, a test is then made in block DML-020 tosee whether there are sufficient number of remaining digits between themost significant digit and the starting position in the partial productto accommodate the adjusted length of the multiplicand. If, for example,three are only three digits between the most significant digit and thestarting digit in the partial product and the multiplicand is 4 digitslong, then there are not sufficient digits in the partial product toaccommodate the addition of the full multiplicand and block DML-020 thenadjusts the length of the multiplicand so that only the number of digitsof the multiplicand will be added to the partial product. By doing this,truncation takes place as the multiply is done. This shortening of themultiplicand which amounts to adjusting its length is done in blockDML-021 if necessary. After the multiplicand is shifted relative to thepartial product, the exit is taken to point MULT in which themultiplicand is then added to the partial product the number of timescorresponding to the value of the current multiplier digit beginningwith block DML-009.

In the current example in which the ten's position of the multiplier isa zero, the branch performed in block DML-010 will immediately result inthe exit being taken to block DML-017 and the hundredths position of themultiplier will then be processed. This will result in block DML-018being entered which will again shift the multiplicand to the leftrelative to the partial product and the hundredths digit of themultiplier will be processed by eventually returning via point MULT toblock DML-009. At this point, when the hundredths position of themultiplier is processed, the pointers in the partial product startingposition will point to the hundredths position in the partial productsuch that the unit's position in the multiplicand will be added to thehundredths position in the partial product. The pointers to the partialproduct in RAM 2 segment 2 will be as follows:

    ______________________________________                                        RAM 2         RAM 2                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                        20            0000                                                            21            0001                                                            22            2015                                                                                      WP2=22                                                                        NP2=1                                               ______________________________________                                    

After the hundredth's digit of the multiplier has been processed byperforming the loop of block DML-010 through block DML-016 three times,the current multiplier digit will have been decremented to zero andblock DML-017 will be entered with the partial product then being asfollows:

    ______________________________________                                        RAM 2         RAM 2                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                        20            0000        Segment 2                                           21            0073                                                            22            2915                                                            ______________________________________                                    

Block DML-017 then tests to see whether all multiplier digits have beenprocessed. In this case, because there is only three multiplier digitsto be processed, it exits to block DML-022. It should be noted at thispoint that by only processing the significant digits in the multiplier,only three digits must be processed and the leading zeros need not beprocessed. In this case, the three leading zeros in the multiplier neednot be processed and this saves considerable time by not having toperform blocks DML-009 through DML-021 for these leading zeros.

Block DML-022 is entered after the multiplication process has beencompleted and the partial product fully developed in segment 2 of RAM 2.Block DML-022 then gets the decimal indicators 85 into microprocessor 30via monitor multiplexer 80 and monitor logic 22. It also clears thecommercial software instruction overflow (OV), sign fault (SF), greaterthan (G), and less than (L) indicators. Block DML-022 then branches toblock DML-023 if an overflow was detected during the multiplicationprocess or branches to block DML-025 if no overflow occurred during themultiplication. If an overflow was detected, block DML-023 is enteredand a branch is taken depending upon whether the overflow trapping isenabled or not. If overflow trapping is enabled, block DML-024 isentered and the commercial software indicators are stored with overflowindicator set. Block DML-024 then branches to the overflow trap routinewhich processes the overflow trap. If the trapping is not enabled onoverflow block DML-023 branches to block DML-025.

Block DML-025 sets the G and L indicators depending upon the sign of theproduct. Block DML-026 is then entered and the partial product in RAM 2segment 2 is transferred to RAM 1 one word at a time. Block DML-027 isthen entered and the product in RAM 1 is then unpacked into the originaloperand 2 field adding the necessary zone field nibbles if the operand 2is string decimal type. This unpacking of the product from RAM 1 intosegment 1 in RAM 2 is done to both add the necessary zone nibbles, if itis string decimal data, and to keep intact the original neighboringnibbles that may occupy words with the most significant and leastsignificant digits of operand 2. At the end of block DML-027, RAM 1which contains the packed product and RAM 2 segment 1 which contains thefinal product in the decimal format of the original operand 2 are asfollows for the example:

    ______________________________________                                        RAM 1       RAM 1                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                         0          0000           Product in packed                                   1          0073           format                                              2          2915                                                              ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        10          3030           Segment 1                                          11          3030                                                              12          3030           Unpacked product                                   13          3030           in string decimal                                  14          3733           format (i.e., with                                 15          3239           zone nibbles.)                                     16          3135                                                              17          2DNN                                                              ______________________________________                                    

After moving the product to segment 1, block DML-027 exits to blockDML-028 which fixes the sign of the product in RAM 2 segment 1 dependingupon whether the sign is negative or positive, which is a function ofwhether the signs of operand 1 and operand 2 are like or unlike. BlockDML-029 is entered and the commercial software instruction indicatorsare then set. Block DML-030 is then entered which moves the result fromRAM 2 segment 1 back via transceivers 97 79 into microprocessor 30 andfrom there into main memory as specified by data descriptor 2. BlockDML-030 then exits to the FETCH routine which fetches the next softwareinstruction and begins its processing.

From the above discussion of the multiplication routine, it can beappreciated that the stripping of the leading zeros from the multiplierand the multiplicand saves significant time within the multiplicationroutine and that by packing the multiplicand, time is also saved in thatzone field nibbles do not have to be skipped during the computation ofthe partial product.

DECIMAL DIVIDE COMMERCIAL SOFTWARE INSTRUCTION

The decimal divide operation performed by the CPU of the preferredembodiment will now be described. One method of performing a decimaldivide which is well known in the art is to initialize a quotient tozero and then continually subtract the denominator from the numeratorusing decimal subtract operations until the difference goes negative andto increment the quotient by one each time the denominator issuccessfully subtracted from the numerator before the difference goesnegative. When this method is used, the unit's position of thedenominator is lined up with the numerator and the subtractions areperformed with as many digits being processed in each subtraction asthere are digits in the longer of the operands, be it the numerator orthe denominator. This process, although it works, can be very slow ifthe denominator goes into the numerator many times.

A better method of performing a decimal divide is shown in the flowchart of FIG. 14A which is a diagram of an improved prior art method. Inthis method, the quotient is developed by first developing the mostsignificant digit of the quotient and then developing lesser significantdigits. This is done by lining up the most significant digit in thedenominator with the most significant digit in the numerator and doing adecimal subtract. If the result of the subtract is not negative, thepresent quotient digit is incremented by one and the subtract isperformed until such time as the difference between the aligneddenominator and the (convert partial numerator) numerator becomesnegative at which time the denominator is added back into the difference(partial numerator) and the quotient digit is stored away. Then, thedenominator is shifted one decimal digit position to the right and theshifted denominator is then subtracted from the partial numerator untilsuch time as the difference (new current partial numerator) becomesnegative at which time the new quotient digit is stored away and thepartial numerator is again made positive by adding back in thedenominator before the denominator is again shifted one position to theright and a new quotient digit is developed by subtracting the shifteddenominator from the partial numerator until such time as the partialnumerator becomes negative.

This process is continued until the unit's position of the partialnumerator is aligned with the unit's position of the denominator atwhich time all quotient digits have been developed and the remainder isthe final difference.

This process greatly speeds up the division over the non-aligned methodpreviously discussed in which the denominator is simply subtracted fromthe numerator until the difference becomes negative. An example of thisprior art method is shown in Table 16 in which a denominator of 0043 isdivided into a numerator of 00006402. As can be seen, the quotientdigits are developed from the most significant digit (Q1) to the leastsignificant digit (Q5). This division results in a quotient of 000148and a remainder of 38.

                  TABLE 16                                                        ______________________________________                                        Prior Art Divide of 00006402 by 0043                                          ______________________________________                                         00006402          Q1=0                                                       -0043                                                                         -9997              Q1=1                                                       +0043                                                                          00006             Q1=0, Q2=0                                                 -00043                                                                        -99963             Q2=1                                                       +00043                                                                         000064            Q2=0, Q3=0                                                 -000043                                                                        000021            Q3=1                                                       -000043                                                                       -999978            Q3=2                                                       +000043                                                                        0000210           Q3=1, Q4=0                                                 -0000043                                                                       0000167           Q4=1                                                       -0000043                                                                       0000124           Q4=2                                                         .                                                                             .                                                                             .                                                                            00000382          Q4=4, Q5=0                                                 -00000043                                                                      00000339          Q5=1                                                         .                                                                             .                                                                             .                                                                           -99999995                                                                     +00000043                                                                         38             Q5=8                                                       ______________________________________                                         Quotient = 00148                                                              Remainder = 38                                                           

An improvement on this method is shown in the flow chart of FIG. 14Bwhich is very similar to the flow chart of 14A except that the leadingzeros of the numerator and the denominator are stripped away so that thealignment of the numerator and the denominator is done on the mostsignificant non-zero digit as opposed to the most significant digit inthe numerator's and denominator's fields as was done in the method ofFIG. 14A. The results of stripping away the leading zeros in thenumerator and denominator is to further increase the speed by which thedivision can be done because the leading zero digits need not beprocessed when performing the subtraction operations. An example of thismethod is shown in Table 17 using the same denominators and numeratorsas used above in Table 16.

                  TABLE 17                                                        ______________________________________                                        Improved Divide                                                               ______________________________________                                         6402             Q1=0                                                        -43                                                                            21               Q1=1                                                        -43                                                                           -78               Q1=2                                                        +43                                                                            210              Q1=1, Q2=0                                                  -043                                                                           167              Q2=1                                                        -043                                                                           124              Q2=2                                                          .                                                                             .                                                                             .                                                                            382              Q2=4, Q3=0                                                  -043                                                                           339              Q3=1                                                          .                                                                             .                                                                             .                                                                           -995              Q3=9                                                        +043                                                                           038              Q3=8                                                        ______________________________________                                         Quotient = 148                                                                Remainder = 38                                                           

The improved method of FIG. 14B also improves the speed by packing thenumerator and denominator so that zone field nibbles are eliminated sothat at the completion of developing the quotient and remainder thequotient and remainder must have leading zeros supplied as necessary aswell as necessary zone field nibbles if the quotient or remainder are inthe string decimal format.

A specific example will now be discussed with the detailed flow chartsof the method in FIG. 15. In the preferred embodiment, the decimaldivide commercial software instruction takes operand 1 as thedenominator and divides it into operand 2, the numerator, and stores thequotient in a field pointed to by data descriptor 3 and stores andremainder back into the field previously occupied by operand 2 which ispointed to by data descriptor 2. In the following example, a numeratorof -0006402 will be divided by a denominator of -0043 to produce aquotient in a field designated as containing 6 digits plus a sign suchthat it will produce a quotient of +000148 and the remainder will goback into the field previously occupied by operand 2 such that theremainder will equal -00000038.

For this example, assume the DDV software instruction is at main memorylocation 1000 as follows:

Example DDV Software Instruction:

    ______________________________________                                        Memory    Memory                                                              Location  Contents                                                            Addressing                                                                              (Hexa-                                                              (Hexadecimal)                                                                           decimal)  Meaning                                                   ______________________________________                                        1000      002B      DDV op code                                               1001      E507      data descriptor 1 (DD1) word 1                            1002      1102      data descriptor 1 (DD1) word 2                            1003      6907      data descriptor 2 (DD2) word 1                            1004      1204      data descriptor 2 (DD2) word 2                            1005      6707      data descriptor 3 (DD3) word 1                            1006      1306      data descriptor 3 (DD3) word 2                            ______________________________________                                    

Data descriptors DD1, DD2 and DD3 are decoded as follows (see FIG. 9):

DD1:

T=0: String (unpacked) decimal.

C1=1: OP 1 starts in right byte.

C2, C3=11: Trailing sign

L=5: 4 digits and sign.

CAS: OP 1 starts in word addressed by contents of base register 7 plusdisplacement of 1102. If B7 contains the value 1000 hexadecimal, OP 1 islocated at address 2102 hexadecimal.

DD2:

T=0: String (unpacked) decimal.

C1=0: OP 2 starts in left byte.

C2, C3=11: Trailing sign.

L=9: 8 digits and a sign.

CAS: OP 2 starts in word addressed by contents of base register 7 plusthe displacement of 1204. Since B7 contains 1000 hexadecimal, OP 2 islocated at address 2204 hexadecimal.

DD3:

T=0: String (unpacked) decimal.

C1=0: OP 3 starts in left byte.

C2, C3=11: Trailing sign.

L=7: 6 digits and a sign.

CAS: OP 3 starts in word addressed by contents of base register 7 plusthe displacement of 1306. Since B7 contains 1000 hexadecimal, OP 3 islocated at address 2306 hexadecimal.

OP 1, which is a -0043 with a trailing minus sign, appears in mainmemory as follows:

    ______________________________________                                        Memory         Memory                                                         Location       Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2102           NN30                                                           2103           3034                                                           2104           332D                                                           ______________________________________                                    

where:

N are neighbor nibbles.

30, 30, 34 and 33 are unpacked decimal digits of 0, 0, 4 and 3respectively.

2D is a trailing minus sign.

OP 2, which is -00006402 with a trailing minus sign, appears in mainmemory as follows:

    ______________________________________                                        Memory         Memory                                                         Location       Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2204           NN30                                                           2205           3030                                                           2206           3036                                                           2207           3430                                                           2208           322D                                                           ______________________________________                                    

where:

N are neighbor nibbles which must be preserved when the remainder isstored.

30, 30, 30, 30, 36, 34, 30 and 32 are unpacked decimal digit of 0, 0, 0,0, 6, 4, 0 and 2 respectively.

2D is an unpacked trailing minus sign.

OP 3, which is to receive the quotient, appears in main memory asfollows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2306           XXXX                                                           2307           XXXX                                                           2308           XXXX                                                           2309           SSNN                                                           ______________________________________                                    

where:

X are don't care nibbles to be overlayed by quotient digits.

S are don't card nibbles to be overlayed by quotient sign.

N are neighbor nibbles which must be preserved.

The execution of the above example, DDV commercial software instruction,will now be described with reference to FIG. 15. FIG. 15 is a flow chartof the firmware microroutines used by CPU 20 to execute a DDV softwareinstruction. The blocks in FIG. 15 which are referred to by the namesnext to them, such as DDV-001, show at a gross level the functionsperformed by microprocessor 30 and commercial instruction logic 28 toperform the software instruction. Some of these blocks may represent theexecution of more than one 48 to 56-bit microinstruction, the form ofwhich is shown in FIG. 5. Before entering the microroutines shown inFIG. 15, which are peculiar to the DDV commercial software instructions,the CPU 20 examines the first word of the software instruction which isbeing executed to determine the type of operation to be performed. Onceit is determined that it is a decimal arithmetic operation as determinedby locking at the operation code in the first word of the instruction,the CPU 20 then proceeds to decode the address syllable associated withdata descriptor 1 to determine the main memory word address and theposition within the word in which operand 1 begins. This front endprocessing of the software instruction then continues with themicroprocessor branching to the DDV routine at block DDV-000.

When the decimal divide routine is entered at DDV-000, it determineswhether this is a first pass in which operand 1 is to be brought intothe CPU or the second pass in which operand 2 is to be brought into theCPU. If it is the first pass, the firmware then branches to blockDDV-001 which fetches operand 1 (the denominator) into RAM 1 aspreviously described for the decimal add instruction. At the end ofblock DDV-001, the contents of RAM 1 are as follows:

    ______________________________________                                                     RAM 1                                                            RAM 1        Contents                                                         Location     (Hexadecimal)                                                    ______________________________________                                        2            NN30                                                             3            3034                                                             4            332D                                                             ______________________________________                                    

Block DDV-001 then exits to block DDV-002 which then strips off leadingzeros of OP 1 as it moves a copy of OP 1 into segment 2 of RAM 2. Thismove and strip of leading zero operation is performed by the subroutinedescribed earlier in the DML instruction.

At the completion of the DDV-002 block, the contents of RAM 2 segment 2will have OP 1 packed with leading zero stripped and zone field nibbleeliminated as follows:

    ______________________________________                                        RAM 2         RAM 2                                                           Location      Contents                                                        (Hexadecimal) (Hexadecimal)                                                   ______________________________________                                        40            43XX        OP 1 packed                                         ______________________________________                                    

After packing the denominator into RAM 2 segment 2 from the copy in RAM1, block DDV-002 then exits to routine IC if an illegal character wasdiscovered during the move operation or exits to the instruction frontend if operand 1 is not equal to zero to prepare to bring in operand 2or it exits to block DDV-003 if operand 1 is equal to zero indicatingthat a divide by zero would be attempted. Block DDV-003 sets thecommercial instruction overflow indicator and exits to the divide byzero trap routine which causes a trap. If an illegal character was notdiscovered in operand 1 and if the operand is not equal to zero, theinstruction front end routine eventually returns to block DDV-000 toperform the second pass in which case block DDV-004 is entered.

Block DDV-004 test whether an immediate operand has been specifiedwithin the divided software instruction for the operand which is to beprocessed next. If an immediate operand has been specified for eitheroperand 2 or operand 3, a branch is taken to a routine IS which is anillegal specification routine which will cause a software trap becauseit is illegal to specify an immediate operand for an operand in whichdata is to be stored. Operand 2 will have the remainder stored in it andoperand 3 will have the quotient stored in it. Therefore, neitheroperand 2 nor operand 3 may be immediate operations. If the next operandhas not been specified to be an immediate operand, block DDv-004branches to block DDV-006 to get the numerator if it is the second passor it branches to block DDV-005 to bring in operand 3 if it is the thirdpass so that the neighbors may be saved when the quotient is stored intothat field.

During the second pass, block DDV-006 is entered and operand 2 which isthe numerator is brought into RAM 1 and also into segment 1 of RAM 2such that at the end of block DDV-006, RAM 1 and RAM 2 segment 1 containthe numerator as follows:

    ______________________________________                                        RAM 1       RAM 1                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                         4          NN30           OP 2 - numerator                                    5          3030                                                               6          3036                                                               7          3430                                                               8          322D                                                              ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        20          NN30           Segment 1                                          21          3030           OP 2 - numerator                                   22          3036                                                              23          3430                                                              24          322D                                                              ______________________________________                                    

Block DDV-007 is then entered and a test of the sign of operand 1 andoperand 2 is made to calculate the resultant quotient sign which will bepositive if the signs are equal, and will be negative if the signs arenot equal, and the resultant quotient sign flag is put into segment 0word 0 of RAM 2 as follows:

    ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        0           0000          Segment 0 word 0                                                              Sign flag of quotient:                                                        0=positive                                                                    FFFF=negative                                       ______________________________________                                    

Block DDV-008 is then entered and the operand 2 is then moved to RAM 2segment 3 stripping the leading zeros and removing the zone fieldnibbles if it is a string operand by copying the copy of operand 2 inRAM 1 into RAM 2 such that at the end of block DDV-008 the contents ofRAM 2 segment 3 are as follows:

    ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        60          6402          Segment 3                                                                     OP 2 numerator                                                                packed with leading                                                           zeros stripped.                                     ______________________________________                                    

Block DDV-008 exits to routine IC if an illegal character was discoveredwhile moving operand 2 and exits to a ZERO PREP routine if operand 2 isequal to zero. A zero numerator yields a zero quotient and a zeroremainder such that the length of the quotient that will be stored inthe field specified by data descriptor 3 is set equal to zero and thelength of the remainder which will be stored in the field specified bydata descriptor 2 is set equal to zero. If operand 2 is not equal tozero as is determined by the zero indicator from decimal indicators 85not being in the equal zero state, then a test is made to compare thelength of the significant digits in operand 2 which is the numerator tothe number of significant digits in operand 1 which is the denominatorto determine whether just on the basis of the number of non-zero digitsin the operands whether the quotient will be greater than zero. If thenumber of non-zero digits in operand 2 is greater than the number ofnon-zero digits in operand 1, then an improper fraction will result suchthat there will be a non-zero quotient and block DDV-009 is entered. Ifthe length of the non-zero field of the operand 2 numerator, is lessthan the length of the non-zero field in the operand 1 denominator, thena proper fraction will result and it is known that the quotient will bea zero such that the length of the quotient is set to zero and thelength of the remainder is set to the length of the data descriptor 2because operand 2 will be the remainder. In this case of a properfraction, the routine then branches to the ZERO PREP routine.

If an improper fraction will result and block DDV-009 is entered, itthen transfers operand 1 from RAM 2 segment 2 to RAM 1 such that RAM 1now contains the stripped and packed version of the denominator asfollows:

    ______________________________________                                        RAM 1       RAM 1                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        0           43XX          OP 1 denominator                                                              packed and stripped.                                ______________________________________                                    

Block DDV-009 then transfers to block DDV-010. Block DDV-010 and blockDDV-011 free up some of the registers within microprocessor 30 bystoring information that is needed later in the divide operation intounused areas of RAM 2. Block DDV-012 then determines the number ofexpected digits in the quotient. This is done by taking the number ofsignificant digits of the operand 2 which is the numerator andsubtracting from it the number of significant digits of operand 1 whichis the denominator and then adding one to the difference. In this case,the expected number of quotient digits is equal to four (which is thelength of the non-zero field of operand 2) minus 2 (which is the lengthof the non-zero field of operand 1) plus 1 which is equal to 3.

Block DDV-013 then sets up the pointers to the version of operand 2which is to be used during the divide. If operand 2 is string data, thepacked version found in RAM 2 segment 3 is used and if operand 2 waspacked data then the original version found in RAM 2 segment 1 is used.After setting up the pointers to operand 2, block DDV-014 is enteredwhich sets the current quotient digit equal to 0. In this case, thefirst time this block is entered, the most significant digit of thequotient is being set equal to zero. This quotient digit is kept in aregister within microprocessor 30 which is incremented each time thesubtraction loop that follows is done.

Block DDV-015 is then entered which initializes the address counter andnibble counter of RAM 1 to point to the unit's position of the operand 1denominator which is stored in RAM 1 such that the address counter willpoint to word 0 and the nibble pointer will point to nibble 1 whichcontains the decimal 3 digit which is the unit's position of thedenominator. Block DDV-016 is then entered and the address counter andnibble counter of RAM 2 are set up to point to the current subtractstarting position of operand 2 in RAM 2 which is used as the currentpartial numerator. In the example case this amounts to pointing to word0 of segment 3 which is word 60 in RAM 2 and to pointing to the digit 4which is in nibble 1 of word 60.

Block DDV-017 is then entered and subtracts the denominator in RAM 1from the current partial numerator in RAM 2 one digit at a time with thenumber of digits being subtracted equal to the non-zero field length ofthe denominator. The first time through block DDV-017, this amounts tosubtracting the unit's digit of the denominator which contains a decimal3 from the hundredths position in the numerator which contains a decimal4 to produce a decimal 1 which replaces the hundredths digit in thepartial numerator in RAM 2. The second time through the loop, the ten'sposition of the denominator which contains a decimal 4 is subtractedfrom the thousandths position in the numerator which contains a decimal6 to produce a decimal 2 and no carry-out. Decimal 2 is stored into theone-thousandth's position in the partial numerator replacing the decimal6. This subtraction of two digits then completes the subtraction of thedenominator on a digit by digit basis from the partial numerator andblock DDV-017 then exits to block DDV-019 because the first quotientdigit is being worked on (i.e., the most significant digit of thequotient). For the second and subsequent quotient digits, block DDV-017exits to block DDV-018 which subtracts a zero from the next digit to theleft in the numerator using the carry-out from the previous subtractionto accommodate a possible carry-out from subtracting the mostsignificant digit of the denominator. This subtraction of individualdenominator digits from numerator digits is performed by amicroinstruction containing a CIPSUB, a CWRES2, a CLDFLP, a CTDCT1 and aCTDCT2 microoperation such that a digit from RAM 1 is subtracted from adigit from RAM 2 and the result in written back into RAM 2 and thecounters to the denominator in RAM 1 are decremented to point to thenext more significant digit in the denominator and the counters in RAM 2are decremented to point to the next more significant digit in thepartial numerator. This subtraction of a zero from the next moresignificant digit within the partial numerator is accomplished by amicroinstruction having a CIPSUB, a CWRES2, a CINOP1 and a CLDFLPmicrooperaiton which results in inhibiting the output of RAM 1 andselecting a zero from RAM 1 zero multiplexer 82 with this zero beingsubtracted from the digit from RAM 2 using the carry from the previousdigit and writing the result back into RAM 2 while updating the decimalindicators 85.

Block DDV-019 is then entered and the current quotient digit isincremented by one. The current quotient digit is kept in a a registerin microprocessor 30 and incremented by one and is only transferred backto the commercial instruction logic after the value of the currentquotient digit has been finally determined. Block DDV-019 then exits toblock DDV-029 if the result of the subtracting the denominator from thepartial numerator is equal to zero as determined by the equal zeroindicator of decimal indicators 85. Block DDV-019 exits to block DDV-020if the equal zero indicator does not indicate that the result is equalto zero and there was a carry-out of the most significant digit asindicated by the carry-out decimal indicator 85. If the result is notequal to zero and there was no carry-out then block DDV-019 goes toblock DDV-015.

Block DDV-015 is then entered to set the address counter and nibblecounter of RAM 1 to point back to the unit's position of the denominatorwhich is stored in RAM 1. Block DDV-016 is then entered and the addresscounter and nibble counter of RAM 2 are then adjusted back to thecurrent subtract starting position so that they point to the leastsignificant digit in the field within the partial numerator which iscurrently being worked on. Thus, at the end of block DDV-016 thepointers in the numerator and the denominator have been reset so thatanother subtract loop can be performed because the previous subtractloop did not yield a zero result and the result did not go negative.Block DDV-016 then returns to block DDV-017 which will subtract thedenominator from the current subtract field within the current partialnumerator. At the beginning of the second time through this subtractloop, the contents of RAM 1 and RAM 2 are as follows:

    ______________________________________                                        RAM 1      RAM 1                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                         0         43XX         OP 2 - Denominator                                                            WP1 = 0                                                                       NP1 = 1                                               ______________________________________                                        RAM 2      RAM 2                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                        60         2102         Segment 3                                                                     OP 1=Partial numerator                                                        WP2 = 60                                                                      NP2 = 1                                               ______________________________________                                    

Before the subtract begins, the current subtract field being worked onwithin the partial numerator contains the result of the previoussubtraction of the denominator from the partial numerator. Afterperforming the subtract on the entire length of the denominator, blockDDV-019 is entered and the current quotient digit of 1 is incremented 2which is equal to the number of times that the denominator has beensubtracted. Block DDV-019 then exits to block DDV-020 because the resultis not equal to zero and there was a carry-out indicating that thedenominator has been subtracted from the partial numerator one more timethan the denominator will go into the partial numerator. Block DDV-020then decrements the current quotient digit whch makes it go from thevalue of 2 to the value of 1 to adjust it for this oversubtraction.

Block DDV-021 is then entered and the RAM 1 pointers are adjusted topoint to the unit's position of the denominator and the RAM 2 pointersare adjusted to point to the current subtract starting position of thepartial numerator. Block DDV-022 is then entered to add back thedenominator to the partial numerator to compensate for theoversubtraction. Block DDV-022 accomplishes this add by performing amicroinstruction containing a CWRES2, a CLDFLD, a CDCT1 and a CDCT2microoperation in a loop which results in adding each digit of thedenominator to its correspondng digit in the partial numerator andplacing the resultant digit back into the partial numerator stored inRAM 2. After adding the entire length of the denominator to the partialnumerator, block DDV-022 then exits to block DDV-023 if it is not thefirst quotient digit which is being worked. Block DDV-023 adds one moredigit by adding a zero with carry from the previous digit to the nextmore significant digit in the partial numerator field to handle the caseof a carry-out from the previous digit. If it is not the first quotientdigit, block DDV-022 exits to block DDV-024 which decrements the currentquotient digit by one to adjust for the oversubtraction. Thus, thecurrent quotient digit is adjusted from 2 to 1 the first time throughthis block.

After adjusting the current quotient digit, block DDV-024 exits to blockDDV-025 if there are more quotient digits to be determined or blockDDV-026 if this was the last quotient digit to be determined (i.e., theleast significant digit of the quotient). In the example case, at thispoint block DDV-024 would exit to block DDV-025 which sets up point ofthe address counter of RAM 2 to point to the position in segment 0 wherethe quotient digits are to be stored. Block DDV-025 then exits to blockDDV-033 which store the current quotient digit into segment 0. In theexample, this results in storing the first quotient digit in word 1 ofsegment 0 of RAM 2. This quotient digit has a value of decimal 1 suchthat at this point segment 0 word 0 contains the sign of the resultindicator as stored earlier and the first quotient digit which is equalto a decimal 1 as follows:

    ______________________________________                                        RAM 2      RAM 2                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                        0          0000         Segment 0                                             1          0001         OP 3-quotient sign flag                                                       1st quotient digit                                    ______________________________________                                    

Block DDV-034 then increments the quotient pointer to point to the nextword in segment 0 for storing the next quotient digit when it isdetermined. Block DDV-035 then adjusts the subtract starting positionwithin the partial numerator to the right by one decimal digit. BlockDDV-035 then exits to block DDV-014 which sets the current quotientdigit, which is a counter within microprocessor 30, equal to 0. BlockDDV-015 and DDV-016 then set up their pointers to the unit's position inthe denominator and the current subtract starting position in thepartial numerator in preparation for starting a new loop which subtractsthe denominator from the partial numerator as many times as it canbefore the result goes negative. When block DDV-016 is exited, thecontents of RAM 1 and RAM 2 in their pointers are as follows:

    ______________________________________                                        RAM 1      RAM 1                                                              Locations  Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                         0         43XX         OP 1=Denominator                                                              WP1=0                                                                         NP1=1                                                 ______________________________________                                        RAM 2      RAM 2                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                        60         2102         OP 1=Partial numerator                                                        WP2=60                                                                        NP2=2                                                 ______________________________________                                    

The subtract loop is then performed starting a block DDV-017 and thequotient digit is determined by subtracting decimal 43 from decimal 210as many times as it will go such that eventually block DDV-033 isentered to store the second quotient digit into the second word ofsegment 0 RAM 2 so that the contents of segment 0 would then be asfollows:

    ______________________________________                                        RAM 2      RAM 2                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                                                Segment 2                                             0          0000         OP3-quotient sign flag                                1          0001         1st quotient digit                                    2          0004         2nd quotient digit                                    ______________________________________                                    

After the pointers are adjusted to the next quotient digit and to thestarting position for the next subtract, the subtract routine is againentered at block DDV-017 and the denominator is again subtracted fromthe partial numerator. Eventually this will result in block DDV-024being entered to decrement the current quotient counter and taking thebranch that goes to block DDV-026 when the last quotient digit is beingprocessed. Block DDV-026 then loads the address counter of RAM 2 withthe address of the word in segment 0 where the last quotient digit is tobe stored. This last quotient digit is the least significant digit ofthe quotient. Block DDV-026 then tests to determine whether the sign ofoperand 2 is a plus sign or a minus sign, and if it is a plus sign,exits to block DDV-027 to set a flag indicating that the remainder whichwill be stored in field previously occupied by operand 2 should have aplus sign and if operand 2 was negative, then block DDV-026 exits toblock DDV-028 to store a flag indicating that the remainder is to have anegative sign. Block DDV-027 and block DDV-028 after setting theremainder sign flag to the proper state then exit to block DDV-032 whichstores the last quotient digit. At this point, the contents of RAM 2segment 0 which contains the quotient digits and the quotient sign flagand segment 3, which contains the remainder of the operand 2 numeratorare as follows:

    ______________________________________                                        RAM 2      RAM 2                                                              Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                                                Segment 0                                             0          0000         OP3=quotient sign flag                                1          0001         1st quotient digit                                    2          0004         2nd quotient digit                                    3          0008         3rd quotient digit                                                            Segment 3                                             60         0038         OP2=numerator/                                                                remainder                                             ______________________________________                                    

Before leaving the discussion of the method by which the quotient digitsare computed, the case in which block DDV-019 takes the branch thatindicates the result of subtracting the denominator from the partialnumerator is zero and the branch is taken to block DDV-029 will bediscussed. In block DDV-029, the number of remaining quotient digits tobe calculated is decremented by one and block DDV-030 is entered whichsets up the address counter of RAM 2 to point to the word in segment 0where the current quotient digit is to be stored. When this path istaken from block DDV-019, it is not necessary to decrement the currentquotient digit because there has been no oversubtraction and it isfurther not necessary to add back the the denominator into the numeratorto compensate for the oversubtraction. Block DDV-030 then exits andtests whether the divide is finished or not depending upon the number ofremaining quotient digits to be calculated as indicated by the quotientdigit counter. If there are more quotient digits to be calculated, blockDDV-030 exits to block DDV-033 and proceeds as described above. If thedivide is finished as indicated by the number of remaining quotientdigits being equal to zero, block DDV-030 exits to block DDV-031. BlockDDV-031 is taken when the remainder is equal to zero because the resultof the subtraction above was zero and the last quotient digit has beenprocessed. Therefore, block DDV-031 sets the sign flag of the remainderto indicate that the remainder has a positive sign. Block DDV-032 isthen entered and the last quotient digit is stored in a word in segment0 of RAM 2. Block DDV-032 then exits to block DDV-036 which testswhether operand 2 is packed decimal data or string decimal data and ifit is packed decimal data goes to block DDV-037 and if it is stringdecimal data goes to block DDV-041.

In the example division, operand 2 is string decimal data so blockDDV-036 exits to block DDV-041 which copies the remainder which is whatis left of the partial numerator in RAM 2 segment 3 to RAM 1 such thatRAM 1 at the end of block DDV-041 is as follows:

    ______________________________________                                        RAM 1       RAM 1                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        0           0038           Packed remainder                                   ______________________________________                                    

After copying the remainder into RAM 1, block DDV-042 then copies andunpacks the remainder from RAM 1 into RAM 2 segment 1 which contains themain memory image of operand 2 which was preserved in order to save theneighboring nibbles. After block DDV-042 unpacks the remainder, RAM 2segment 1 is as follows:

    ______________________________________                                        RAM 2     RAM 2                                                               Location  Contents                                                            (Hexadecimal)                                                                           (Hexadecimal)                                                       ______________________________________                                                                 Segment 1                                            20        NN30           Unpacked remainder                                   21        3030                                                                22        3030                                                                23        3033                                                                24        382D                                                                ______________________________________                                    

In this example, since the packed remainder only contained 4 digits,only the first four least significant digits of the remainder areunpacked into segment 1 of RAM 2 by block DDV-042 and the leading fourdigits within segment 1 of RAM 2 are those which are left over from themain memory image of the original operand 2. Block DDV-043 is thenentered to add the zone field nibbles with leading zeros as is necessaryto completely fill out the remainder in RAM 2 segment 1. In this casethe execution of block DDV-043 does not result in any change to thecontents of segment 1 because the 4 leading digit positions within theoperand 2 were initially zeros. These were written over with leadingzeros by block DDV-043 also such that at the end of block DDV-043 thecontents of segment 1 of RAM 2, which is the main memory image of theremainder, are as above. Block DDV-043 then exits to block DDV-038.

If the operand 2 was not a string decimal data type field, block DDV-036would have exited to block DDV-037 to adjust the pointers to point tothe leading sign position of operand 2 as stored in segment 1 of RAM 2such that when block DDV-038 is entered from either block DDV-037 orblock DDV-043 the address counter and nibble pointers of RAM 2 willpoint to the first nibble within the operand 2 field. Block DDV-038 isthen executed to fix the sign of the remainder in the OP 2 field. BlockDDV-038 also gets the commercial software instruction indicators andclears the overflow (OV), specification fault (SF), greater than (G) andless than (L) indicators. Block DDV-039 then stores the commercialsoftware indicators. Block DDV-040 is then entered to set up for pass 3of the instruction which brings in operand 3 and sets it up inpreparation for storing the quotient into the operand 3 field. BlockDDV-040 then exits to the instruction front end processing routine whichdoes some preliminary processing upon data descriptor 3 and eventuallyreturns to block DDV-003 which will, on the third pass, result in blockDDV-005 being entered.

In block DDV-005, operand 3 field is brought into RAM 2 segment 4 tosave the neighboring nibbles before the quotient is written into thefield and the field written back into main memory. Block DDV-004 getsthe quotient sign flag from RAM 2 segment 0 word 0. Block DDV-045 setsthe RAM 1 address counter to point to word 0 and the nibble counter topoint to nibble 3. Block DDV-046 is then entered to take the quotientstored in RAM 2 segment 0 one digit at a time starting with the leastsignificant digit and transferring it into RAM 2 segment 4 whichcontains the operand 3 field, and to supply the necessary zone bits andleading zeros as is necessary. If the operand 3 field length is greaterthan the number of quotient digits stored in RAM 2 segment 0. Thiscopying of the quotient from segment 0 of RAM 1 to segment 4 of RAM 2 isdone in blocks DDV-046 through DDV-056 and at the end of which RAM 2segment 4, which contains the quotient with the proper sign, appears asfollows:

    ______________________________________                                        RAM 2     RAM 2                                                               Location  Contents                                                            (Hexadecimal)                                                                           (Hexadecimal)                                                       ______________________________________                                                                 Segment 4                                            80        3030           Unpacked quotient of                                 81        3031           +000148 where "SS"                                   82        3438           will be occupied later                               83        SSNN           by plus sign.                                        ______________________________________                                    

After DDV-059 completes the supplying of leading zeros and zone fieldnibbles as is necessary if there are leading zeros of if the operand 3field is an unpacked decimal field, block DDV-060 is entered to getcommercial software instruction indicators. Block DDV-061 then clearsall indicators but the truncation (TR) indicator bit. Block DDV-061 thenexits to block DDV-062 if the quotient is not equal to zero or to blockDDV-069 if the quotient is equal to zero. If block DDV-062 is entered, atest is made to determine whether an overflow occurred during the divideoperation. If an overflow occurred, block DDV-062 exits to block DDV-064which tests whether traps are enabled and if traps are enabled, blockDDV-066 is entered which sets the commercial instruction overflowindicator bit and then exits to the routine to handle the overflow trap.If traps are not enabled, block DDV-064 exits to block DDV-065. BlockDDV-065 then tests the sign flag of the quotient which was previouslystored.

If the quotient is positive, block DDV-067 is entered and the greaterthan (G) indicator bit is set. If the sign of the quotient is negative,block DDV-068 is entered and the less than (L) indicator bit is set.Block DDV-069 is the entered to fix the sign of the quotient in RAM 2segment 4. Block DDV-070 is then entered which stores the commercialinstruction indicators. Block DDV-071 then proceeds to transfer thequotient from RAM 2 segment 4 into main memory by reading a word at atime from RAM 2 into microprocessor 30 and from there having it storedin a word of main memory. In block DDV-072 the starting address and thelength of the remainder are restored to pointers and counters from wherethey were saved in RAM 2 segment 7 words 0-2. In block DDV-073 theremainder is transferred from RAM 2 segment 1 into main memory by takinga word at a time from RAM 2 into microprocessor 30 and writing it intomain memory. Block DDV-073 completes the processing of the decimaldivide instruction and exits to the routine which fetches the nextsoftware instruction from main memory in preparation for its execution.

As can be appreciated from the above discussion of the decimal divideinstruction, the ability to address the numerator and the denominatorfrom the most significant digit to the least significant digit is madeuse of to strip lodading zeros from both which results in greatlyreducing the number of cycles which must be executed when performing adivide operation.

CONVERT BINARY TO DECIMAL COMMERCIAL SOFTWARE INSTRUCTION

The binary to decimal conversion operation performed by the CPU of thepreferred embodiment will now be described. One method of converting anumber in a binary format to a number is a decimal format which is wellknown in the art is to set an initial decimal partial sum to zero and toplace the binary number in a register which can be shifted such thateach bit within the binary number can be examined. The binary number isthen examined beginning at the most significant bit position and thepartial sum is then doubled by performing a decimal add of the partialsum to itself and adding in the bit being examined as a carry input intothe least significant decimal digit of the partial sum. This process isthen repeated scanning from the more significant to the leastsignificant bits within the binary number until each bit has beenexamined. The decimal partial sum is added to itself each time a bitwithin the binary number is examined with the carry into the leastsignificant digit of the decimal partial sum being set equal to the bitof the binary number that is being examined. Although this processworks, it can be time consuming because the decimal add of the partialsum must be performed for each digit within the binary number beingconverted such that if a binary number is a 32-bit binary number, 32decimal adds of the partial sum must be performed. These additions canbe quite long if the addition is performed on a partial sum that has asmany decimal digits in it as required to hold the largest possiblenumber represented by the length of the binary number (i.e., 2 to 31power plus a sign bit).

In the CPU of the preferred embodiment, an improved method is used whichfirst strips off the leading zeros within the binary number such that ifa binary number is represented in 16 bits and the most significant 8bits are all zeros, the 8 first bits are stripped off thereby reducingthe total number of decimal additions which need to take place on thepartial sum. Secondly, the length of the partial sum is initially setequal to one decimal digit and the length is adjusted only as the numberof digits within the partial sum increases so that each addition of thepartial sum only is required to do a decimal add on as many digits asrequired to hold the partial sum at any given instance.

A specific example of a binary to decimal conversion operation will nowbe discussed with respect to the detailed flow charts of the method asshown in FIG. 16. In the preferred embodiment, the binary to decimalconversion commercial software instruction takes the binary number givento it as operand 1 and it converts it to a decimal number and stores theresult in the field specified for operand 2. In the following example, abinary number of -37 will be converted to a decimal number which is tobe placed in a packed decimal field with a trailing sign. In thepreferred embodiment, negative binary numbers are represented in a two'scomplement form.

For the example binary to decimal conversion shown below, assume the CBDsoftware instruction is at main memory location 1000 as follows:

Example CBD Software Instruction:

    ______________________________________                                        Memory    Memory                                                              Location  Contents                                                            Addressing                                                                              (Hexa-                                                              (Hexadecimal)                                                                           decimal)  Meaning                                                   ______________________________________                                        1000      0027      CBD op code                                               1001      0207      data descriptor 1 (DD1) word 1                            1002      1102      data descriptor 1 (DD1) word 2                            1003      6487      data descriptor 2 (DD2) word 1                            1004      1204      data descriptor 2 (DD2) word 2                            ______________________________________                                    

Data descriptors DD1 and DD2 are decoded as follows (see FIG. 9):

DD1:

T=0: String (binary DD is a string DD).

C1=0: OP 1 starts in left byte.

L=2: 16-bit binary operand.

CAS: OP 1 starts in word addressed by contents of base register 7 plusdisplacement of 1102. If B7 contains the value 1000 hexadecimal, OP 1 islocated at address 2102 hexadecimal.

DD2:

T=1: Packed decimal.

C1,C2=01: OP 2 starts in nibble 1.

C3=1: Trailing sign.

L=4: 3 digits and a sign.

CAS: OP 2 starts in word addressed by contents of base register 7 plusthe displacement of 1204. Since B7 contains 1000 hexadecimal, OP 2 islocated at address 2204 hexadecimal.

OP 1, which is a -37 in two's complement form, appears in main memory asfollows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2102           FFDB                                                           ______________________________________                                    

OP 2, which is where the converted number is to be stored, appears inmain memory as follows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2204           NXXX                                                           2205           SNNN                                                           ______________________________________                                    

where:

N are neighbor nibbles which must be preserved when the converted numberis stored.

X are where the decimal digits of the converted number are to be stored.

S is where the sign of the converted number is to be stored.

The execution of the above example CBD commercial software instructionwill now be described with reference to FIG. 16. FIG. 16 is a flow chartof the firmware microroutines used by CPU 20 to execute a CBD softwareinstruction. The blocks in FIG. 16 which are referred to by the namesnext to them, such as CBD-001, show at a gross level the functionsperformed by microprocessor 30 and commercial instruction logic 28 toperform the software instruction. Some of these blocks may represent theexecution of more than one 48 or 56-bit microinstruction, the form ofwhich is shown in FIG. 5. Before entering the microroutines shown inFIG. 16, which are peculiar to the CBD commercial software instructions,the CPU 20 examines the first word of the software instruction which isbeing executed to determine the type of operation to be performed. Onceit is determined that it is a decimal arithmetic operation as determinedby looking at the operation code in the first word of the instruction,the CPU 20 then proceeds to decode the address syllable associated withdata descriptor 1 to determine the main memory word address and theposition within the word in which operand 1 begins. This front endprocessing of the software instruction then continues with themicroprocessor branching to the CBD routine at block CBD-000.

When the binary to decimal conversion routine is entered at CBD-000, itdetermines whether this is a first pass in which operand 1 is to bebrought into the CPU or the second pass in which operand 2 is to bebrought into the CPU. If it is the first pass, the firmware thenbranches to block CBD-001 which fetches operand 1 (the binary number tobe converted) into microprocessor 30 During the second pass, blockCBD-003 is entered and the operand 2 so that it can be shifted andexamined one bit at a time starting with the most significant bit. BlockCBD-001 then exists to the instruction front end routine whicheventually returns to block CBD-000 to perform the second pass in whichcase block CBD-002 is entered. Block CBD-002 analyzes the datadescriptor 2 to determine the starting and end positions of the operand2 field which is to hold the converted number and the length of thefield. Block CBD-003 then brings in operand 2 so that the neighbors maybe saved when the converted number is stored into that field in mainmemory. Operand 2, which is the field in which the converted number isto be stored, is brought into segment 1 of RAM 2 such that at the end ofblock CBD-003, RAM 2 segment 1 contains the converted number field asfollows:

    ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        20          NXXX           Segment 1                                          21          SNNN           OP 2 - field to                                                               receive converted                                                             number.                                            ______________________________________                                    

Block CBD-004 is then entered and a test of the sign of operand 1 binaryis made. If the sign of the binary number to be converted is negative,block CBD-004 branches to block CBD-005 to perform a two's complement onthe binary operand and to note that the sign of the result should benegative. Block CBD-005 then exits to block CBD-006 which sets up theappropriate bit counter to equal either 16-bits, if a single precisionbinary number containing 16 bits is to be converted, or equal to 32, ifa double precision number containing 32 bits is to be converted. BlockCBD-006 then exits to block CBD-009. If block CBD-005 determines thatthe binary number to be converted is a positive number, block CBD-007 isentered and a flag is set to indicate that the result is positive. BlockCBD-007 then exits to block CBD-008 which sets up a bit counter equal to16, if it is a single precision binary number to be converted, to 32, ifa double position binary number is to be converted.

Block CBD-009 is then entered and the binary number withinmicroprocessor 30 is then shifted to the left until the first non-zerobinary bit is encountered and the length of the binary field which hasto be converted, either 16 or 32, is adjusted to reflect the number ofnon-zero bits remaining in the binary number. If block CBD-009determines that all bits within the binary number to be converted arezeros, block CBD-009 exits to the zero result routine, which is routineCBD-020. If the binary number to be converted is not zero, block CBD-009after stripping off all leading zero bits then exits to block CBD-010which puts a binary ZERO into the unit's position of the operand 2 fieldwhich will contain the converted binary number in a decimal format. Atthe end of block CBD-010, the operand 2 field in RAM 2 segment 1 is asfollows:

    ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        20          NXX0            Segment 1                                         21          SNNN            OP 2, 1 digit                                                                 partial sum of 0.                                 ______________________________________                                    

Block CBD-011 then exits to ADD BIT, which is block CBD-019. BlockCBD-019 then sets the carry-out indicator within decimal indicators 85such that when the addition of the partial sum to itself is performed,the carry-out indicator will have a binary ONE in it which will be usedas the carry-in when the unit's digit iof the partial sum is added toitself during the decimal partial sum doubling operation. During thefirst time through, the carry-out indicator can be unconditionally setto indicate a carry-out because binary number to be converted in blockCBD-009 such that it is known that the bit currently being worked on atthe left end of the binary number is a binary ONE. After setting thecarry-out indicator within decimal indicators 85, block CBD-019 thenexits to the DOUBLE routine at block CBD-012.

In block CBD-012 the current decimal partial sum is doubled by adding itto itself. The carry-in bit, of the unit's position of the partial sumis set equal to the bit within the binary number which is currentlybeing converted. In block CBD-012 this doubling of the decimal partialsum is done by initializing the address counter and nibble counter topoint to the word and nibble containing the unit's digit within thepartial sum. In the example case, the word address counter is set equalto 20 which is word 0 of segment 1 and the nibble counter is set equalto 3 so that it points to the third nibble which is the unit's positionin the partial sum. Also, the decimal indicators 85 are initialized suchthat the illegal indicator is set equal to zero and the equal nine andequal zero indicators are set equal to binary ONEs. This is done byperforming a CRESET microoperation. The carry-out indicator of decimalindicators 85 is set equal to the binary state of the bit which is beingexamined in the binary number and will be set equal to a binary ONE ifthe bit is a binary ONE and will be set equal to a binary ZERO if thebit is a binary ZERO.

A microinstruction is then executed which contains a CIPDUB, a CWRES2, aCLDFLD, and a CTDCT2 microoperation. These microoperations have theeffect of bringing the nibble pointed to by the address and nibblecounters of RAM 2 out of RAM 2 nibble multiplexer 89 and through doublemultiplexer 83 into the A port of decimal ALU 84 and the same nibblefrom RAM 2 out of RAM 2 zero multiplexer 90 and into the B port ofdecimal ALU 84 and from there the decimal digit 4-bit result is writtenback into the same nibble within RAM 2 and the indicator bits fromdecimal ALU 84 are stored in decimal indicators 85. Then the nibblecounter within nibble write control 86 is then decremented by one topoint to the next more significant digit within RAM 2 which contains thedecimal partial sum and the address counter is decremented by one if thenibble counter decrements through 0. This microinstruction is repeateddepending upon the number of active digits there are in the partial sum.The first time through block CBD-012 there will only be one digit activewithin the partial sum so that this microinstruction is only executedone time. The ability of the preferred embodiment to feed both the A andthe B ports of decimal ALU 84 from the output of RAM 2 provides a veryefficient method of doubling the partial sum contained in RAM 2. At theend of the first time that block CBD-012 is executed, the partial sumstored in RAM 2 segment 1 is as follows:

    ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        20          NXX1          Segment 1                                           21          SNNN          OP 2, 1 digit partial                                                         sum of 1.                                           ______________________________________                                    

It should be noted that the doubling of the partial sum done in blockCBD-012 also includes a microinstruction to write and skip the zonenibbles if the operand 2 is of the string decimal type such that theyhave zone nibbles equal to a binary 0011.

After adding all digits in the decimal partial sum, block CBD-012 exitsto block CBD-013 which branches depending upon whether there is anydigits left within operand 2 field length which are not currently beingused in the partial sum. This branch is done by examining a counterwhich contains a value equal to the length of operand 2 minus thecurrent length of the partial sum. If the length of the partial sum isless than the length of the operand 2 block CBD-013 branches to blockCBD-027 which tests whether there was a carry-out of the mostsignificant digit when the decimal partial sum was added to itself. Ifthere was no carry-out, then block CBD-027 branches to block CBD-028. Ifthere was a carry-out of the most significant decimal digit of thepartial sum, block CBD-027 branches to block CBD-029. In block CBD-029 adecimal one is written into the next more significant digit in thedecimal partial sum by feeding decimal ALU 84 with a decimal zero fromRAM 1 zero multiplexer 82 into the A port and by feeding the B port witha decimal zero from RAM 2 zero multiplexer 90 while adding in the binaryONE as a carry-in. This writing of a decimal 1 in the next moresignificant digit of the decimal partial sum is performed by amicroinstruction containing CINOP1, CINOP2, CWRES2, CLDLFP and CTDCT2microoperations which write the out of decimal ALU 84 into RAM 2. BlockCBD-029 then increments the length of the decimal partial sum so thatthe increased length will be used the next time the decimal partial sumis added to itself.

Block CBD-030 then decrements the count of available digits by 1 so thata comparison can be made in block CBD-013 to determine if there is anyroom left in operand 2 to expand the decimal partial sum by 1 digit.Block CBD-030 then exits to block CBD-027 which then decrements thecount of bits in the binary number that remains to be converted. BlockCBD-016 is then entered and shifts the binary number one position to theleft to move the next lesser significant bit into position to beconverted. Upon exiting, block CBD-016 branches to block CBD-017, if thebit counter indicates that there are more bits to convert, or to blockCBD-020, if all bits have been converted. If there are more bits toconvert, block CBD-017 sets up the counter that indicates the number ofdecimal digits in the partial sum so that counter can be used whendoubling the decimal partial sum. Block CBD-017 then branches to blockCBD-018, if the current bit to be converted in the binary number is abinary ZERO, or to block CBD-019 if the current bit to be converted is abinary ONE.

Block CBD-018 resets the carry-out indicator of decimal ALU 84 to abinary ZERO because the current bit to be converted is a binary ZERO.Block CBD-019 sets the carry-out indicator of decimal ALU 84 to a binaryONE because the current bit to be converted is a binary ONE. BlocksCBD-018 and CBD-019 both exit to block CBD-012 which doubles the decimalpartial sum as discussed above and adds in the carry-out indicator intothe unit's position.

After doubling the partial sum, block CBD-012 then exits to blockCBD-013 to again test if there are any unused digits left within theoperand 2 field length. This test branches as indicated before and theprocess is continued until all bits within the binary number have beenconverted. If, during this conversion process, the length of the partialsum reaches the length of the operand 2 as indicated by the availabledigit counter being ZERO, block CBD-013 will branch to block CBD-014. Inblock CBD-014 the bit counter which indicates the counter of binary bitsyet to be converted is decremented by one and then a test is made to seewhether there was a carry-out of the most significant bit of the partialsum. If there was no carry-out, the block CBD-014 exits to blockCBD-016. If, however, there was a carry-out of the most significantdigit of the decimal partial sum, an overflow condition exists becauseone or more digit is needed in the decimal partial sum to hold thecarry-out of the most significant digit in the partial sum and blockCBD-015 sets an overflow indicator which is checked later. Block CBD-016is then entered and the next bit within the binary number beingconverted is shifted into position and the process continued. When blockCBD-016 determines that all bits have been converted, it branches toblock CBD-020.

Block CBD-020 then fills the rest of the operand 2 field from the mostsignificant digit in the partial sum through the most significant digitof operand 2 with leading zeros (and zone nibbles as necessary if theoperand 2 is the string decimal type). Block CBD-020 is also entered ifit was earlier determined that the binary number being convertedcontained all binary zeros such that the converted number will containall decimal zeros. Upon leaving block CBD-020, the converted number asstored in RAM 2 segment 1 is as follows:

    ______________________________________                                        RAM 2       RAM 2                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        20          N037          Segment 1                                           21          SNNN          OP 2 - final partial                                                          sum with sign yet                                                             to be put in place.                                 ______________________________________                                    

Block CBD-021 is then entered and the commercial instruction indicatorsare gotten and the overflow (OV) and sign fault (SF) indicator bits arecleared. A branch is then made to determine whether an overflowcondition was encountered during the binary conversion process such thatthe result in RAM 2 only contains the truncated value with the mostsignificant digits being truncated. If the overflow indicator is set,block CBD-021 branches to block CBD-022 which sets the overflowindicator in the commercial instruction indicators and does a branchdepending upon whether traps are enabled or not. enabled, block CBD-023is entered and the commercial instruction indicators are stored with theoverflow indicators set.

If traps are not enabled or if there was no overflow during the binaryconversion process, block CBD-024 is entered and the sign of theconverted decimal number is set into the sign nibble within the operand2 field in segment 1 of RAM 2. At the end of CBD-024, the contents ofRAM 2 segment 1, which is the final converted decimal number are asfollows:

    ______________________________________                                        RAM 2     RAM 2                                                               Location  Contents                                                            (Hexadecimal)                                                                           (Hexadecimal)                                                       ______________________________________                                        20        N037           Segment 1                                            21        DNNN           OP 2-final converted                                                          number with trailing                                                          minus sign (-037).                                   ______________________________________                                    

Block CBD-025 is then entered and the commercial instruction indicatorsare stored in their hardware register.

Block CBD-026 is then entered and the converted binary number that hasbeen converted to the decimal format and stored in segment 1 of RAM 2with its neighbor nibbles is now transferred into main memory into theoperand 2 field as described by data descriptor 2. This transfer is doneone word at a time by moving a word from RAM 2 into main memory with themain memory address being supplied by microprocessor 30.

From the above description of the convert binary to decimal softwareinstruction, it can be appreciated that the stripping of leading zerobits in the binary number before beginning the conversion process cangreatly reduce the number of times in which the decimal partial sum mustbe added to itself. Further, by using a decimal partial sum which has alength only sufficiently long to hold the current decimal partial sum,the number of decimal digits which must be added when doubling thepartial sum is greatly reduced. The length of the partial sum isincreased by one digit each time there is a carry out of the mostsignificant digit of the decimal partial sum. The preferred embodimenttakes further advantage of the commercial instruction logic 28 by usingthe ability to feed both the A and B ports of the decimal ALU 84 withthe same operand thereby eliminated the necessity of copying the partialsum from the place where the sum is stored to the other memory so thatboth the memory holding the sum and the other memory can be fed into theB and A ports respectively.

CONVERT DECIMAL TO BINARY COMMERCIAL SOFTWARE INSTRUCTION

The decimal to binary conversion operation performed by the CPU of thepreferred embodiment will now be described. The method used in the CPUof the preferred embodiment is basically to bring in the number to beconverted which is in a decimal format into the commercial instructionlogic 28 and to examine each decimal digit one digit at a time startingwith the most significant digit. Before starting to examine the mostsignificant decimal digit, a binary partial sum is set equal to zero.This binary partial sum is kept in microprocessor 30 which has anarithmetic logic unit which performs binary arithmetic. Then as eachdecimal digit in the number to be converted is examined, beginning withthe most significant digit, the digit is added into the current binarypartial sum after the binary partial sum is multiplied by ten. Thisprocess continues until all decimal digits have been examined and thecomplete binary partial sum has been calculated.

A specific example of a decimal to binary conversion operation will nowbe discussed with respect to the detailed flow charts of the method asshown in FIG. 17. In the preferred embodiment, the decimal to binaryconversion commercial software instruction takes the decimal numbergiven to it as operand 1 and it converts it to a binary number andstores the result in the field specified for operand 2. In the followingexample, a decimal number of -000123 with a trailing sign will beconverted to a binary number which is to be placed in a double precision(32 bits) binary field. In the preferred embodiment, negative decimalnumbers are represented in a two's complement form.

For the example decimal to binary conversion shown below, assume the CDBsoftware instruction is at main memory location 1000 as follows:

Example CDB Software Instruction:

    ______________________________________                                        Memory    Memory                                                              Location  Contents                                                            Addressing                                                                              (Hexa-                                                              (Hexadecimal)                                                                           decimal)  Meaning                                                   ______________________________________                                        1000      002A      CDB op code                                               1001      E707      data descriptor 1 (DD1) word 1                            1002      1102      data descriptor 1 (DD1) word 2                            1003      8407      data descriptor 2 (DD2) word 1                            1004      1204      data descriptor 2 (DD2) word 2                            ______________________________________                                    

Data descriptors DD1 and DD2 are decoded as follows (see FIG. 9):

DD1:

T=0: String (unpacked) decimal.

C1=1: OP 1 starts in right byte.

C2,C3=11: Trailing sign.

L=7: 6 digits and a sign.

CAS: OP 1 starts in word addressed by contents of base register 7 plusdisplacement of 1102. If B7 contains the value 1000 hexadecimal, OP 1 islocated at address 2102 hexadecimal.

DD2:

T=0: String (binary DD is a string DD).

C1=1: OP 2 starts in left byte.

L=4: 32-bit binary operand.

CAS: OP 2 starts in word addressed by contents of base register 7 plusthe displacement of 1204. Since B7 contains 1000 hexadecimal, OP 2 islocated at address 2204 hexadecimal.

OP 1, which is a -000123 in string decimal format, appears in mainmemory as follows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2102           NN30                                                           2103           3030                                                           2104           3132                                                           2105           332D                                                           ______________________________________                                    

where:

N are neighbor nibbles which must be preserved when the converted numberis stored.

30, 31, 32, 33 are string decimal digits 0, 1, 2, 3 respectively.

2D is a trailing minus sign.

OP 2, which is where the converted number is to be stored, in a binaryformat appears in main memory as follows:

    ______________________________________                                        Memory                                                                        Location       Memory                                                         Address        Contents                                                       (Hexadecimal)  (Hexadecimal)                                                  ______________________________________                                        2204           MSWD                                                           2205           LSWD                                                           ______________________________________                                    

where:

MSWD is 16-bit word which will contain the 16 most significant bits ofthe converted 32-bit binary number.

LSWD is 16-bit word will will contain the 16 least significant bits ofthe converted 32-bit binary number.

The execution of the above example CDB commercial software instructionwill now be described with reference to FIG. 17. FIG. 17 is a flow chartof the firmware microroutines used by CPU 20 to execute a CDB softwareinstruction. The blocks in FIG. 17 which are referred to by the namesnext to them, such as CDB-001, show at a gross level the functionsperformed by microprocessor 30 and commercial instruction logic 28 toperform the software instruction. Some of these blocks may represent theexecution of more than one 48 or 56-bit microinstruction, the form ofwhich is shown in FIG. 5. Before entering the microroutine shown in FIG.17, which are peculiar to the CDB commercial software instructions, theCPU 20 examines the first word of the software instruction which isbeing executed to determine the type of operation to be performed. Onceit is determined that it is a convert decimal to binary operation asdetermined by looking at the operation code in the first word of theinstruction, the CPU 20 then proceeds to decode the address syllableassociated with data descriptor 1 to determine the main memory wordaddress and the position within the word in which operand 1 begins.

The front end processing of the software instruction then continues withthe microprocessor branching to the CDB routine at block CDB-000. Whenthe convert decimal to binary routine is entered at CDB, it isdetermined whether this is a first pass in which operand 1 is to bebrought into the CPU or the second pass in which the converted number inbinary format is to be stored into the operand 2 field. If it is thefirst pass, the firmware then branches to block CDB-001 which fetchesoperand 1. so that it can be examined one decimal digit at a timestarting with the most significant decimal digit.

After block CDB-001 has brought the decimal number to be converted intoRAM 1, the contents of RAM 1 are as follows:

    ______________________________________                                        RAM 1       RAM 1                                                             Location    Contents                                                          (Hexadecimal)                                                                             (Hexadecimal)                                                     ______________________________________                                        2           NN30          OP 1, the string                                    3           3030          decimal number to                                   4           3132          be converted                                        5           332D                                                              ______________________________________                                    

Block CDB-002 is then entered which sets up the RAM 1 address counterand nibble counter to the most significant digit of the decimal numberto be converted in RAM 1. The pointers are set to point to the nibblecontaining the decimal value and not to the nibble containing the zonebits of binary 0011. In the example, the address counter is set up topoint to word 2 and the nibble counter is set to point to nibble 3 whichcontains the most significant digit which is a decimal 0. Block CDB-002then exits to block CDB-003 where a counter is set equal to the lengthof the number of digits in the decimal number to be converted. BlockCDB-003 also zeros out the word zero within RAM 2 so that when thedecimal digit to be converted is written into nibble 3 of RAM 2 from RAM1 the 12 leading bits with be binary ZEROs.

Block CDB-004 is then entered to bring the current digit pointed to bythe address and nibble counters of RAM 1 into microprocessor 30 throughRAM 2. This is done by reading the current nibble out of RAM 1 throughRAM 0 multiplexer 82 and double multiplexer 83 into the A port ofdecimal ALU 84 while feeding the B port of the decimal ALU 84 with azero from RAM 2 zero multiplexer 90. The output of decimal ALU 84 istaken from the result/zone multiplexer 91 and into nibble 3 multiplexer95 where it is written into word 0 nibble 3 in RAM 2. The word 0 of RAM2 is then read into RAM 2 data register 88 and from there it is taken bytransceivers 97 into microprocessor 30. The decimal digit transferredform RAM 1 into microprocessor 30 is then stored in the leastsignificant word of a 32-bit binary partial sum with the mostsignificant word of the 32 bit binary partial sum set equal to zero.After reading the current decimal digit from RAM 1 into RAM 2, a test ismade within block CDB-004 to determine if a string decimal number isbeing converted and, if so, the nibble counter for RAM 1 is incrementedby one by use of a microinstruction containing a CTUCT1 microoperationwhich increments the nibble counter and, if the nibble counterincrements through 3, also increments the address counter of RAM 1.

After the decimal digit has been transferred to microprocessor 30, blockCDB-005 is entered and a test is made on the status of the equal zeroindicator of decimal indicators 85. If the equal zero indicatorindicates that a decimal zero digit was read from RAM 1, block CDB-005transfers back to block CDB-004 to get the next digit, if all digitshave not already been transferred. If block CDB-005 determines that allthe digits are zero and that no more digits are left, because a decimaloperand length counter has been decremented to zero, block CDB-005 exitsto block CDB-006 which sets the converted binary result equal to zerobecause all of the digits within RAM 1 have been scanned and they wereall zeros. When block CDB-005 determines that a non-zero decimal digithas been read from RAM 1 into microprocessor 30, block CDB-007 isentered. Block CDB-007 tests whether there are any digits left withinRAM 1 which have not yet been converted by examining the status of thedecimal operand length counter to determined whether it has beendecremented to zero. If there are still digits left to be converted, thebinary partial sum within microprocessor 30 is multiplied by 10 withcare being taken to note if there is any overflows out of the mostsignificant bit in the 32-bit binary number. This multiplication by tenis done by shifting the binary partial sum one bit position to the leftto multiply the binary partial sum by two. The by-two result is savedand then shifted two more places to the left to effectively multiply bythe binary partial sum by eight and then add the by-two partial sum isadded to it to produce a binary partial sum which has been multiplied byten.

Block CDB-010 is then entered to get the next digit from RAM 1 intomicroprocessor 30 via RAM 2 as done before and the values in the nibbleand address counter of RAM 1 are incremented to point to the next lesssignificant digit in the digital number being converted. Block CDB-011is then entered and the current decimal digit entered intomicroprocessor 30 is then added to the binary partial sum with careagain being taken to note any overflow out of the most significant bitof the 32-bit binary partial sum which is being accumulated. BlockCDB-011 then returns to block BDC-007 which tests the counter indicatingthe number of decimal digits remaining to be converted. If the count isnot zero, block CDB-011 exits to block CDB-009 which multiplies thebinary partial sum by 10 as before. This process is continued from blockCDB-007 through CDB-011 until all digits within the decimal number to beconverted have been processed. When the counter remaining digits isequal to zero, block CDB-007 exits to block CDB-008. In block CDB-008 atest is made on decimal illegal decimal indicator of decimal indicators85 (which is an integrating indicator as mentioned previously) todetermine whether any of the digits read from RAM 1 into themicroprocessor via RAM 2 contained a non-decimal digit. If so, blockCDB-008 exits to the illegal character (IC) routine which handles thecase of an illegal digit.

If no illegal digits were found during the conversion process, blockCDB-008 exits to the software instruction front end routine which doesthe preprocessing on data descriptor 2 and returns to the convertdecimal to binary routine at block CDB-000 which tests whether this ispass 1 or pass 2, if pass 2 goes to block CDB-012. Block CDB-012examines the data descriptor 2 and determines the length of operand 2.If data descriptor 2 is an immediate operand type, instead of a pointerto operand 2, a block CDB-012 branches to routine IS which handlesillegal software instructions because immediate operands are not allowedto be used for operand which have results stored in them. If it is notan immediate operand, block CDB-012 exits to block CBD-013 CDB-013 whichchecks to see if the results can be stored within the one or two wordsspecified for operand 2 taking into account that the binary result mayhave to have a two's complement performed on it if the result isnegative. If the result will not fit within the one or two wordsspecified for operand 2, an overflow indicator is set. In block CDB-014,the commercial instruction indicators are read and updated to setoverflow if necessary. If overflow has occurred, and traps are enabled,block CDB-014 branches to the trap overflow routine (OV). If there isnot an overflow or if trapping on overflow has not been enabled, blockCDB-014 exits to block CDB-015 which performs a two's complement on thebinary result if the result is negative as determined by the sign on theoriginal decimal number which was converted. Block CDB-016 is thenentered and the binary result consisting of either two bytes, if it is a16-bit binary number, or 4 bytes, if it is a 32-bit binary number, asspecified by data descriptor 2 are then stored from the registers inmicroprocessor 30 one byte at a time into main memory in order that theneighboring bytes in main memory will be preserved if the binary operanddoes not begin in the right byte of a word. After CDB-016 stores theresults into main memory, the main memory area occupied by the convertedbinary number for the example contains the binary result of -123 intwo's complement form as follows:

    ______________________________________                                        Memory     Memory                                                             Location   Contents                                                           (Hexadecimal)                                                                            (Hexadecimal)                                                      ______________________________________                                        2204       FFFF         -123 in binary                                        2205       FF85         two's complement format.                              ______________________________________                                    

Block CBD-016 then goes to the FETCH routine which brings in the nextsoftware instruction for processing.

This conversion process of converting a decimal number to a binarynumber can be appreciated by looking at Table 18 which shows selectedsteps in the decimal to binary conversion process. In Table 18, thecolumn labeled "step", contains the last three digits of the flow chartblock numbers of the FIG. 17. Under the columns labeled "RAM 1"counters, the "WP1" column indicates where address counter of RAM 1 ispointing at the beginning of the step, and the column labeled "NP1"indicates where the nibble counter of RAM 1 is pointing to at thebeginning of the step. The columns labeled "Decimal Digit from RAM 1",contain the decimal digit which is being examined in RAM 1 in both ahexadecimal and binary format. The columns labeled "Partial Sum"indicate the least significant portion of the binary partial sum whichis being accumulated in microprocessor 30. The partial sum column in thebinary format contains the 8 low order bits of the 32-bit partial sumand the column labeled "Decimal" contains the decimal equivalent.

                                      TABLE 18                                    __________________________________________________________________________    Example Decimal to Binary Conversion                                               RAM 1                                                                    Step Counters                                                                            Decimal Digit from RAM 1                                                                    Partial Sum                                          (CDB-)                                                                             WP1                                                                              NP1                                                                              (Hexadecimal)                                                                         (Binary)                                                                            (Binary)        (Decimal)                            __________________________________________________________________________    004  2  3  0       0000  X X X X X X X X X                                    004  3  1  0       0000  X X X X X X X X X                                    004  3  3  0       0000  X X X X X X X X X                                    004  4  1  1       0001  0 0 0 0 0 0 0 1  1                                   009  4  3                0 0 0 0 1 0 1 0  10                                  010  4  3  2       0010  0 0 0 0 1 0 1 0  10                                  011  5  1                0 0 0 0 1 1 0 0  12                                  009  5  1                0 1 1 1 1 0 0 0 120                                  010  5  1  3       0011  0 1 1 1 1 0 0 0 120                                  011  5  3                0 1 1 1 1 0 1 1 123                                  015                      1 0 0 0 0 1 0 1 123                                  __________________________________________________________________________

From the above discussion of the decimal to binary conversion softwareinstruction, it can be appreciated that in a preferred embodiment use ismade of the ability of RAM 1 to address from left to right such that thedecimal number being converted can be examined starting at the mostsignificant digit and the work to the least significant digit. Thisability is used to first strip all leading zeros from the decimal numberand then to convert from the first most significant non-zero digit tothe least significant digit within the decimal number. It can also beappreciated that the integrating nature of the equal zero and illegalcharacter indicators within decimal indicators 85 is advantageouslyemployed.

While a preferred embodiment has been described, other modificationswill be readily suggested by those of ordinary skill in the art. Forexample, the commercial instruction logic can be adapted to work onwords that have fewer or more than 16 bits and decimal data formatsusing differe9t representations with different nibbles and atom sizes.Also, although the preferred embodiment has been described in terms of aparticular microprocessor, the commercial instruction logic can be usedwith the CPU having different microprocessors or combinatorial logic. Inaddition, the control of the commercial instruction logic can be doneusing different microoperations or combinatorial logic. Similarly, themethods used perform the various arithmetic operations can be adapted touse different hardware.

While the invention has been shown and described with reference to thepreferred embodiment thereof, it will be understood by those skilled inthe art that the above and other changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A decimal arithmetic logic unit comprising: anarithmetic processor having,a first input port for receiving decimaloperands, a second input port for receiving decimal operands, a carry-inport for receiving a carry out from a previous decimal arithmeticoperation, and an output port for providing a result decimal operandequal to the result of an arithmetic operation performed on decimaloperands received at said first and said second input ports; a firstmultiplexer having an output connected to said first input port, havinga first input for receiving a first decimal operand, having a secondinput for receiving a second decimal operand, and having a select inputfor receiving a first select signal which determines if said firstdecimal operand or said second decimal operand is to be connected to theoutput of said first multiplexer, and a second multiplexer having anoutput connected to said second input port, having a first input forreceiving said second decimal operand, having a second input forreceiving a decimal zero operand, and having a select input forreceiving a second select signal which determines if said second decimaloperand or said decimal zero operand is to be connected to the output ofsaid second multiplexer, whereby said second operand may be applied viasaid first and said second multiplexers respectively to said first inputport and to said second input port of said arithmetic processor so thatsaid second operand may be added to itself when it is desired to convertsaid second operand in decimal format to binary format, and wherebyeither said first or said second decimal operands can be applied viasaid first multiplexer to said first input port of said arithmeticprocessor while said zero operand is connected via said secondmultiplexer to said second input port of said arithmetic processor sothat they may be subtracted from said zero operand when it is desired tocomplement either said first or said second operand in decimal format.2. The apparatus of claim 1 wherein said arithmetic processor furtherhas an arithmetic operation input for receiving an operation signalwhich when in a first state indicates that a decimal subtract is to beperformed by taking the operand at said first input port from theoperand at said second input port, and said operation signal when in asecond state indicates that a decimal addition is to be performed byadding the operand at said first input port to the operand at saidsecond input port, whereby said second decimal operand can be added toitself by having sid operation signal in said second state and saidfirst and said second select signals causing said first and said secondmultiplexers to connect said second decimal operand to both said firstand said second input ports.
 3. The apparatus of claim 2 furthercomprising a third multiplexer having an output connected to said firstinput of said first multiplexer, said third multiplexer having a firstinput for receiving said first decimal operand, having a second inputfor receiving a decimal zero operand, and having a select input whichdetermines if said first decimal operand or a decimal zero operand is tobe connected to the output of said third multiplexer.